Bump version to 7.13.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.12.0...497ea0475dc2d01a5a7d15893f1adba43a36bd3a

• Release notes:

### New scenarios and scenario updates

- !777 Add StagingRef 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:

  • Update the release notes for the newly created tag.