Add branches-ignore to merge_group schema

[lrotschy]

See comment on previous PR: #12 (comment)

Investigate adding branches-ignore to merge_group schema.

[lrotschy]

muzimuzhi/hello-github-actions#9

[muzimuzhi]

Just to re-confirm before I report similar issues to github/docs and rhysd/actionlint, does this mean on.merge_group.branches-ignore is officially supported by GitHub Actions in public beta?