Bump version to 6.21.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.20.1...9faf45e2b897e321ac053496c22ad8c1530c9c13

• Release notes:

### Other changes (CI, backstage)

- !640 Set application_settings_cache_seconds to 0 to disable the cache.

• 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.