Bump version to 6.18.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.18.0...bcbed568d3bab5373d5e27a59c118ba310823c39

• Release notes:

### Fixes

- !628 Reverts mTLS test and associated changes, which led to multiple QA pipeline failures

• 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.