Update version to 8.15.1

Diff

https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.15.0...d07d682276820cd2877d3277bf61890121c050bb

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.