Bump version to 6.9.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.8.0...42d685cf14ad252e673f3d9d3927493dc9b9b160

• Release notes:

### Fixes

- !566 Fix failure issue title and labels
- !567 Fix failing specs

### Other changes (CI, backstage)

- !563 Add CI_PROJECT_NAME environment variable
- !571 Clarify note about listing env vars

• 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:

  • Create a tag for the new release version.