Update version to 8.15.1

Diff

v8.15.0...d07d6822

Release notes:

Fixes

• Do not set github mocking on by default (!1113 - merged)
• Revert "Merge branch 'jmd-bug-fix-gitalycluster... (!1114 - merged)

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.