Bump version to 7.15.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v7.15.0...1c275bfd86d32ba31085269b026e9d4059c7e603

• Release notes:

### Fixes

- !799 Add parameter to elasticsearch docker

• 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.