Bump version to 6.21.4

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.21.3...c19eae0fcef801e872e336afd3414af0e4475256

• Release notes:

### Fixes

- !654 Increase availablity timeout

• 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.