Bump VERSION to 6.2.1

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v6.2.0...468cb9006aa14322e80fdb40647500ce61eef343

• Release notes:

### Fixes

- !85 Add relevant requires to individual `.yml` files.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/styles/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.