Bump VERSION to 4.12.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v4.11.0...1ffdf491b3bfe013076846b655eded9025d76307

• Release notes:

### New scenarios and scenario updates

- !333 Update Praefect config
- !329 Revert "Disable Rack::Attack throttling by default in Component::Gitlab"

### Other changes (CI, backstage)

- !331 Add praefect manual jobs
- !330 Add env vars to pass to the QA framework to simulate a slow connection

• 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.