Bump version to 5.15.0

• Diff: v5.14.1...9a94ed2c
• Release notes:

### New scenarios and scenario updates

- !471 Update Praefect scenario to include Gitaly storage.

### Fixes

- !487 No need to login to docker if we not going to pull.
- !488 Fix for Sanity issue 225162

### Other changes (CI, backstage)

- !470 Add options to skip teardown and tests.

• 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.