Bump verison to 6.18.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.17.1...41d8f2a63bfd9056464a01ea4392df0f25bcab66

• Release notes:

### New scenarios and scenario updates

- !616 Add mTLS scenario.

### Other changes (CI, backstage)

- !617 Add SSHTunnel scenario to the pipeline.

• 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.