chore(deps): bump json_serializable

Bumps the json_serializable group in /packages/amplify/amplify_flutter with 1 update: [json_serializable](https://github.com/google/json_serializable.dart).


Updates `json_serializable` from 6.8.0 to 6.9.0
- [Release notes](https://github.com/google/json_serializable.dart/releases)
- [Commits](google/json_serializable.dart@json_serializable-v6.8.0...json_serializable-v6.9.0)

---
updated-dependencies:
- dependency-name: json_serializable
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: json_serializable
...

Signed-off-by: dependabot[bot] <[email protected]>

packages/amplify/amplify_flutter/pubspec.yaml

@@ -34,4 +34,4 @@ dev_dependencies:
   build_runner: ^2.4.9
   flutter_test:
     sdk: flutter
-  json_serializable: 6.8.0
+  json_serializable: 6.9.0