Bump version to 6.15.2

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.15.1...715d0feed794d6ed910d443c3c5445538a6f42cc

• Release notes:

### Fixes
- !608 Fix prepare stage reports when there is no path match.
- !607 Limit the non-running of E2E tests for release commits to the canonical project.

• 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.