Bump version to 5.4.0

• Diff: v5.3.0...2cf63a0e

• Release notes:

### New scenarios and scenario updates

- !416 Remove OAuth scenario

### Other changes (CI, backstage)

- !411 Label quarantined test reports

• 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.