Bump version to 5.4.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v5.3.0...a9572dd36374041454e9afee529b7d1cd78d78dc

• Release notes:

### New features and features updates

- !65 Add deprecate use of CopHelper cop.

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/styles/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Update the release notes for the newly created tag.