Bump version to 8.13.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v8.13.0...451954c85a90416de1113829ea0f462660bf6ce3

• Release notes:

### Fixes

- !1087 Fix bug in config for prometheus_listen_addr

• 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.