Group triage packages to use stage label instead of Group label

It’s valid to have several group labels but only one stage label 🤔 Right now these issues will appear in both groups’ triage package but may not be relevant to both groups… (example: https://gitlab.com/gitlab-org/gitlab-ee/issues/9216). Maybe we should find group issues based on the stage label instead? Because we recommend only one of those be added to any issue at one time