Bump VERSION to 6.0.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/-/compare/v5.4.0...b667edf
• Release notes:

### New features and features updates

- !67 Update RuboCop and ancillary gems.

### Doc changes

- !68 Clear up link used for deprecation of cop helper.

• 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.