Bump version to 6.18.2

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.18.1...cd42b06338db66a29f472c9765557d9bcdb02be0

• Release notes:

### Fixes

- !631 Change the default diff ratio threshold to detect existing QA failure issues from 5% to 15%

• 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.