Bump version to 8.15.3

Diff

v8.15.2...d908f831

Release notes

Fixes

• Create Runtime::Env.mock_github_enabled? method (!1123 - 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.