[battery] Bump platform version to NNBD stable (flutter#3587)

Author: stuartmorgan-g

packages/battery/battery_platform_interface/CHANGELOG.md

@@ -1,4 +1,4 @@
-## 2.0.0-nullsafety
+## 2.0.0
 
 * Migrate to null safety.
 

packages/battery/battery_platform_interface/pubspec.yaml

@@ -3,20 +3,20 @@ description: A common platform interface for the battery plugin.
 homepage: https://github.com/flutter/plugins/tree/master/packages/battery
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 2.0.0-nullsafety
+version: 2.0.0
 
 dependencies:
   flutter:
     sdk: flutter
-  meta: ^1.3.0-nullsafety
-  plugin_platform_interface: ^1.1.0-nullsafety.1
+  meta: ^1.3.0
+  plugin_platform_interface: ">=1.0.0 <3.0.0"
 
 dev_dependencies:
   flutter_test:
     sdk: flutter
   mockito: ^5.0.0-nullsafety.0
-  pedantic: ^1.10.0-nullsafety
+  pedantic: ^1.10.0
 
 environment:
-  sdk: ">=2.12.0-0 <3.0.0"
-  flutter: ">=1.9.1+hotfix.4"
+  sdk: ">=2.12.0-259.9.beta <3.0.0"
+  flutter: ">=1.20.0"