Bump version to v12.1.0

Diff

https://gitlab.com/gitlab-org/ruby/gems/gitlab-styles/-/compare/v12.0.1...2fa60bb535410290175651d5d7e2a8da529e95f7

Checklist

• Change the VERSION constant to a minor version in lib/gitlab/styles/version.rb (you might have to change the version number in the next steps according to SemVer).
• Ensure the diff link above is up-to-date.
• Add release notes to the Changelog section below.
• Based on the diff and the release notes, update the version.rb according to SemVer.
• Create an MR on gitlab-org/gitlab project with the New Version of gitlab-styles.md template to test the new version of gitlab-styles, and follow the MR instructions.

Changelog