Bump version to 6.15.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.15.0...308c913cd54512b9f945c77a276e5a87bee1dfbf

• Release notes:

### Fixes

- !604 Dont run e2e jobs on release pipelinees.

• 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.