Use caret syntax with flutter create command (#150920)

The Pull Request changes the `dartSdkVersionBounds` value, which gets templated into the `pubspec.yaml` from the `flutter create`:

https://github.com/flutter/flutter/blob/ffdeaa1995efc1fe7d759e13b92eba1ec57beea6/packages/flutter_tools/templates/app/pubspec.yaml.tmpl#L8

As from the Dart dependencies ["Best practices"](https://dart.dev/tools/pub/dependencies#use-caret-syntax) section of the documentation:

> Use caret syntax
Specify dependencies using the [caret syntax](https://dart.dev/tools/pub/dependencies#caret-syntax). This allows the pub tool to select newer versions of the package when they become available. Further, it places an upper bound on the allowed version.

To learn more about the Caret syntax, refer to the [Caret syntax documentation](https://github.com/flutter/flutter/blob/ffdeaa1995efc1fe7d759e13b92eba1ec57beea6/packages/flutter_tools/templates/app/pubspec.yaml.tmpl#L8).

**Additional context**
- VGVentures/io_crossword#592 (comment) (cc: @kevmoo)
- [Very Good Templates](https://github.com/VeryGoodOpenSource/very_good_templates) relies on the Caret syntax when templating the Dart SDK

Author: alestiago

packages/flutter_tools/lib/src/commands/create.dart

@@ -339,7 +339,7 @@ class CreateCommand extends CreateBase {
       linux: includeLinux,
       macos: includeMacos,
       windows: includeWindows,
-      dartSdkVersionBounds: "'>=$dartSdk <4.0.0'",
+      dartSdkVersionBounds: '^$dartSdk',
       implementationTests: boolArg('implementation-tests'),
       agpVersion: gradle.templateAndroidGradlePluginVersion,
       kotlinVersion: gradle.templateKotlinGradlePluginVersion,

packages/flutter_tools/test/commands.shard/permeable/create_test.dart

@@ -3446,6 +3446,27 @@ void main() {
     expect(pubspec.description, 'a: b');
   });
 
+  testUsingContext('should use caret syntax in SDK version', () async {
+    await _createProject(
+      projectDir,
+      <String>[
+        '--no-pub',
+      ],
+      <String>[
+        'pubspec.yaml',
+      ],
+    );
+
+    final String rawPubspec = await projectDir.childFile('pubspec.yaml').readAsString();
+    final Pubspec pubspec = Pubspec.parse(rawPubspec);
+
+    expect(
+      pubspec.environment!['sdk'].toString(),
+      startsWith('^'),
+      reason: 'The caret syntax is recommended over the traditional syntax.',
+    );
+  });
+
   testUsingContext('create an FFI plugin with ios, then add macos', () async {
     Cache.flutterRoot = '../..';