Bump version to 5.4.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v5.4.0...3371a37f087e2e85088d667c0fecb59a91ee0efa

• Release notes:

### Fixes

- !421 Fix reporter commenting on wrong discussions.

### Other changes (CI, backstage)

- !419 Set default image, tags, cache and before_script in CI config.

• 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:

  • Create a tag for the new release version.