Bump version to 6.15.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.14.0...9a1757d45dd0e0e154bae5b259556eecdd08f3b8

• Release notes:

### Fixes

- !602 Resolve "Handle environment names from deployer".

### Other changes (CI, backstage)

- !601 Add auto-release to the CI config.

• 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.