Bump version to 6.21.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.21.0...2622b174d44a225301ea0ca50dc22282662c6397

• Release notes:

### Fixes

- !645 Workaround to detach from Docker containers

### Doc changes

- !642 Add transient bug test docs

### Other changes (CI, backstage)

- !644 Remove .travis.yml

• 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.