Bump version to 6.6.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.5.0...bbab737bdf3efcb01030689b78384d511ef1536a

• Release notes:

### New scenarios and scenario updates

- !548 Add actioncable scenario

• 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.