Bump VERSION to 6.6.0

• Diff: https://gitlab.com/gitlab-org/gitlab-styles/compare/v6.5.0...a1c47f58aec298dd7bb0b40d65ef5c61efa85030

• Release notes:

### New features and features updates

- !97 Add `Performance/Rubyzip` Cop.

### Other changes (tooling, technical debt)

- !99 Update gem's URL in gitlab-styles.gemspec.

• 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.