build: upgrade Dart SDK version to 3.9.0

- Update SDK version in pubspec.yaml from ^3.8.0 to ^3.9.0
- Ensure compatibility with the latest Dart SDK version

Author: fulleni

pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/flutter-news-app-full-source-code/auth-repository
 publish_to: none
 
 environment:
-  sdk: ^3.8.0
+  sdk: ^3.9.0
 
 dependencies: 
   auth_client: