Bump version to 7.9.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v7.9.0...3e58f9cd9a76633f30db27e64f14b4232f50e590

• Release notes:

### Other changes (CI, backstage)

- !760 Update gitlab-styles to latest version

• 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:

  • Update the release notes for the newly created tag.