chore(deps): bump build_runner

Bumps the build_runner group with 1 update in the /packages/amplify/amplify_flutter directory: [build_runner](https://github.com/dart-lang/build).


Updates `build_runner` from 2.5.4 to 2.8.0
- [Release notes](https://github.com/dart-lang/build/releases)
- [Commits](dart-lang/build@build_runner-v2.5.4...build_runner-v2.8.0)

---
updated-dependencies:
- dependency-name: build_runner
  dependency-version: 2.8.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: build_runner
...

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

Author: dependabot[bot]

packages/amplify/amplify_flutter/pubspec.yaml

@@ -34,4 +34,4 @@ dev_dependencies:
   build_runner: ^2.4.9
   flutter_test:
     sdk: flutter
-  json_serializable: ">=6.9.4 <6.10.0"
+  json_serializable: ^6.9.4