Bump version to 6.21.2

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.21.1...18c0bbb516dfbd667b813d11b8450198340bc38e

• Release notes:

### Fixes

- !647 Fix omnibus settings for registry enabling

### Other changes (CI, backstage)

- !646 Sanity monday hours
- !550 Use a single commit API call in version sanity check

• Checklist before merging:

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

• Checklist after merging:

  • Update the release notes for the newly created tag.