Bump version to 7.13.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.13.0...9de8e9b9fb528b80b71ee7ffa40a495fdeb89824

• Release notes:

### Fixes

- !780 Fix invalid anchor

### Other changes (CI, backstage)

- !775 Exclude most jobs when triggered by feature flag definition changes.
- !781 Update Staging Ref URL

• 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.