Bump version to 6.12.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.11.0...663a49ae2233d87d7a63a6bd5caab98821facc0e

• Release notes:

### Other changes (CI, backstage)

- !590 Allow jobs to include GitLab QA CLI options.
- !591 Add GITLAB_QA_USER_AGENT env variable.

• 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.