Update downstream QA jobs to use main branches

What does this MR do?

Update the CI config to use the main downstream QA project branches. This MR has been created by cherry-picking required commits from !8 (closed)

@dsearles, Please let me know if this looks good. Otherwise, we can merge it.

What are the relevant issue numbers?

gitlab-org/gitlab#299585 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer