Bump version to 6.1.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.0.0...f71a6d8bacb662e2b0c6d9482d39614239890dc2

• Release notes:

### Other changes (CI, backstage)

- !507 Find testcases by `status_issue` tag

• 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:

  • Create a tag for the new release version.