Bump json_serializable in /experimental/web_dashboard (#1105)

Bumps [json_serializable](https://github.com/google/json_serializable.dart) from 6.1.5 to 6.1.6.
- [Release notes](https://github.com/google/json_serializable.dart/releases)
- [Commits](https://github.com/google/json_serializable.dart/compare/json_serializable-v6.1.5...json_serializable-v6.1.6)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

experimental/web_dashboard/pubspec.lock

@@ -386,7 +386,7 @@ packages:
       name: json_serializable
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "6.1.5"
+    version: "6.1.6"
   lints:
     dependency: transitive
     description: