Bump version to 6.22.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.21.4...5244e8e4aa0c33ae7dcc86cc7e51c2759d287b66

• Release notes:

### Doc changes

- !657 Update instructions for testing a custom image.

### Other changes (CI, backstage)

- !658 Switching to using a root certificate

• 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.