Bump version to 4.0.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/-/compare/v3.4.0...bump-version-to-4.0.0

• Release notes:

### New scenarios and scenario updates

- !38 Update Rubocop to 0.82.0.

• 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:

  • Create a tag for the new release version.

/label Engineering Productivity ~backstage