Bump json_serializable from 6.7.1 to 6.8.0

Bumps [json_serializable](https://github.com/google/json_serializable.dart) from 6.7.1 to 6.8.0.
- [Release notes](https://github.com/google/json_serializable.dart/releases)
- [Commits](google/json_serializable.dart@json_serializable-v6.7.1...json_serializable-v6.8.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

pubspec.lock

@@ -269,18 +269,18 @@ packages:
     dependency: "direct main"
     description:
       name: json_annotation
-      sha256: b10a7b2ff83d83c777edba3c6a0f97045ddadd56c944e1a23a3fdf43a1bf4467
+      sha256: "1ce844379ca14835a50d2f019a3099f419082cfdd231cd86a142af94dd5c6bb1"
       url: "https://pub.dev"
     source: hosted
-    version: "4.8.1"
+    version: "4.9.0"
   json_serializable:
     dependency: "direct dev"
     description:
       name: json_serializable
-      sha256: aa1f5a8912615733e0fdc7a02af03308933c93235bdc8d50d0b0c8a8ccb0b969
+      sha256: ea1432d167339ea9b5bb153f0571d0039607a873d6e04e0117af043f14a1fd4b
       url: "https://pub.dev"
     source: hosted
-    version: "6.7.1"
+    version: "6.8.0"
   lints:
     dependency: transitive
     description: