Update year2023 flag deprecation message (#162607)

Fixes [Improve `year2023` flag deprecation
message](flutter/flutter#162606)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: TahaTesser

packages/flutter/lib/src/material/progress_indicator.dart

@@ -343,7 +343,8 @@ class LinearProgressIndicator extends ProgressIndicator {
     this.stopIndicatorRadius,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.26.0-0.1.pre.',
     )
     this.year2023,
@@ -420,7 +421,8 @@ class LinearProgressIndicator extends ProgressIndicator {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.1.pre.',
   )
   final bool? year2023;
@@ -719,7 +721,8 @@ class CircularProgressIndicator extends ProgressIndicator {
     this.constraints,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.1.pre.',
     )
     this.year2023,
@@ -749,7 +752,8 @@ class CircularProgressIndicator extends ProgressIndicator {
     this.constraints,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -832,7 +836,8 @@ class CircularProgressIndicator extends ProgressIndicator {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/progress_indicator_theme.dart

@@ -49,7 +49,8 @@ class ProgressIndicatorThemeData with Diagnosticable {
     this.trackGap,
     this.circularTrackPadding,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -131,7 +132,8 @@ class ProgressIndicatorThemeData with Diagnosticable {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/slider.dart

@@ -239,7 +239,8 @@ class Slider extends StatefulWidget {
     this.autofocus = false,
     this.allowedInteraction,
     @Deprecated(
-      'Use SliderTheme to customize the Slider appearance. '
+      'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.1.pre.',
     )
     this.year2023,
@@ -573,7 +574,8 @@ class Slider extends StatefulWidget {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use SliderTheme to customize the Slider appearance. '
+    'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.1.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/slider_theme.dart

@@ -298,7 +298,8 @@ class SliderThemeData with Diagnosticable {
     this.thumbSize,
     this.trackGap,
     @Deprecated(
-      'Use SliderTheme to customize the Slider appearance. '
+      'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -637,7 +638,8 @@ class SliderThemeData with Diagnosticable {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use SliderTheme to customize the Slider appearance. '
+    'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;