Bump version to 7.9.0

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/-/compare/v7.8.5...bba3f55da9c802c4fae1b04e7c37a3b69f9028eb

• Release notes:

### New CLI parameter

- !744 Allow all scenarios to run with feature flags set

• 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.