chore(deps): Bump the built_value_generator group

Bumps the built_value_generator group in /packages/smithy/smithy with 1 update: [built_value_generator](https://github.com/google/built_value.dart).

- [Release notes](https://github.com/google/built_value.dart/releases)
- [Changelog](https://github.com/google/built_value.dart/blob/master/CHANGELOG.md)
- [Commits](google/built_value.dart@v8.6.1...v8.7.0)

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

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

packages/smithy/smithy/pubspec.yaml

@@ -30,7 +30,7 @@ dependencies:
 dev_dependencies:
   amplify_lints: ">=3.0.0 <3.1.0"
   build_runner: ^2.4.0
-  built_value_generator: 8.6.1
+  built_value_generator: 8.7.0
   json_serializable: 6.7.1
   stack_trace: ^1.10.0
   test: ^1.22.1