Update version to 7.21.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.21.0...aac86ca32e287598f9a07ff3884e825f5a98642c

• Release notes:

### Other changes (CI, backstage)

- !843 Report metrics for all containers but telegraf.
- !842 Add test reporting when using combined gitlab-qa options.

• 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.