fix: Rename manifest.yml to manifest.yaml

[carlqt]

What is the purpose of this pull request?

RBS gem expects manifest.yaml instead of manifest.yml. This subtle behaviour causes the users of this library to get UnknownTypeName when validating types.

check https://github.com/soutaro/steep/issues/1103 for more details

What changes did you make? (overview)

Rename manifest.yml to manifest.yaml

Is there anything you'd like reviewers to focus on?

Checklist

• [ ] I've added tests for this change
• [ ] I've added a Changelog entry
• [ ] I've updated a documentation

[palkan]

Thanks! Will craft a new patch release soon.