Bump json_serializable from 4.1.4 to 6.6.2

Bumps [json_serializable](https://github.com/google/json_serializable.dart) from 4.1.4 to 6.6.2.
- [Commits](google/json_serializable.dart@json_serializable-v4.1.4...json_serializable-v6.6.2)

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

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