build: update SDK constraint to ^3.9.0

- Update Dart SDK constraint in pubspec.yaml from ^3.8.0 to ^3.9.0

Author: fulleni

pubspec.yaml

@@ -4,7 +4,7 @@ repository: https://github.com/flutter-news-app-full-source-code/kv-storage-serv
 publish_to: none
 
 environment:
-  sdk: ^3.8.0
+  sdk: ^3.9.0
 
 dev_dependencies:
   mocktail: ^1.0.4