Bump version to 6.7.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.6.0...0c88b75fb94f899fa24df13276b01ada80b5a234

• Release notes:

### Other changes (CI, backstage)

- !552 Update to Ruby v2.7.2

• 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.