chore(deps): update dependency dart to v3

Author: renovate[bot]

pubspec.yaml

@@ -6,7 +6,7 @@ repository: https://github.com/Flutter-Dart-Open-Source/flutter_disposebag.git
 issue_tracker: https://github.com/Flutter-Dart-Open-Source/flutter_disposebag/issues
 
 environment:
-  sdk: '>=2.12.0 <3.0.0'
+  sdk: '3.9.4'
 
 dependencies:
   meta: ^1.3.0