Update version to 9.1.0

• Diff: v9.0.0...4f371cb0

• Release notes:

### Doc changes

- !1130 Clairify purpose of GITLAB_QA_ACCESS_TOKEN`
- !1127 Add gitlab-qa Release Process Diagram

### Other changes (CI, backstage)

- !1128 Allow environment URLs to be overridden 
- !1129 Introduce new dedicated tokens for tooling-related automations

• 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.