Bump version to 7.18.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v7.17.1...8932901b07e8800d99d9857e68979114b683705a

• Release notes:

### New scenarios and scenario updates

- !827 Add cloud activation scenario.

### Other changes (CI, backstage)

- !828 Ignore empty environment variables.
- !812 Add Logger to GitLab QA.

• 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.