chore(deps): bump built_value_generator

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


Updates `built_value_generator` from 8.10.1 to 8.12.0
- [Release notes](https://github.com/google/built_value.dart/releases)
- [Changelog](https://github.com/google/built_value.dart/blob/master/CHANGELOG.md)
- [Commits](https://github.com/google/built_value.dart/commits)

---
updated-dependencies:
- dependency-name: built_value_generator
  dependency-version: 8.12.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: built_value_generator
...

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

Author: dependabot[bot]

packages/smithy/smithy/pubspec.yaml

@@ -31,6 +31,6 @@ dev_dependencies:
   amplify_lints: ">=3.1.3 <3.2.0"
   build_runner: ^2.4.9
   built_value_generator: ^8.10.1
-  json_serializable: ">=6.9.4 <6.10.0"
+  json_serializable: ^6.9.4
   stack_trace: ^1.10.0
   test: ^1.22.1