chore(deps): bump built_value_generator

Bumps the built_value_generator group in /packages/smithy/smithy_aws 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_aws/pubspec.yaml

@@ -30,7 +30,7 @@ dev_dependencies:
   built_value_generator: ^8.10.1
   file: ^7.0.1
   glob: ^2.1.0
-  json_serializable: ">=6.9.4 <6.10.0"
+  json_serializable: ^6.9.4
   pubspec_parse: ^1.2.2
   test: ^1.22.1
   yaml: ^3.1.1