Bump version to 8.15.3

Diff

https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.15.2...d908f831496b5df8c625d3bdf5b30a5c89bb9dd7

Release notes

Fixes

• https://gitlab.com/gitlab-org/gitlab-qa/-/merge_requests/1123+

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.