Bump version to 6.15.4

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.15.3...2c0a2600f9bdb18afb1116c10895e0c28011726a

• Release notes:

### Fixes

- !612 Allow detection of issues that have an empty stacktrace

• 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.