Update Subscriptions that were created as Gold/Silver past EoA

Summary

This is a follow up of this issue: now that Gitlab Subscriptions are created with actual ultimate/premium plan codes (and not silver/gold codes any longer).

We now need to update the existing subscriptions which were created post EoA date with one of the two deprecated plan codes, this is what this migration in this MR will do. But before we do so, we need to make sure that not a single new subscription is created with a deprecated plan:

• the license support team already adjusted the plan codes on their side.
• the team providing Education licenses needs to do the same on their side, as at least one recent Education license was granted to a Gold subscription.