build: update SDK version to 3.9.0

- Upgrade Dart SDK version from 3.8.0 to 3.9.0 in pubspec.yaml

Author: fulleni

pubspec.yaml

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