Remove the trial boolean qualification for ultimate trials
What does this MR do and why?
Remove the trial boolean qualification for ultimate trials
- replace with a plan check, similar to our eligibility check in Namespace.eligible_for_trial where we need to allow previous legacy trials to have another duo enterprise trial with ultimate and the trial boolean never gets set to false on downgrade.
- see https://gitlab.com/gitlab-org/gitlab/-/issues/500325
Changelog: other EE: true
Related to #500325