Sync up new persisted plan between gitlab.com and CustomersDot

Summary

This is the follow-up of #300546 (closed): in that issue, we delivered a short-term solution that shows new plan names (eg. Premium (Formerly Silver)) on gitlab.com for renewed/upgraded/new subscriptions (MR here: !53473 (merged))

Improvements

We need a long-term solution in order to sync up properly new plans between CustomersDot and gitlab.com.

• we need actual records for the new plans in the plans table: !53465 (merged)
• we need to add constants related to the new plans in the Plan class, and implement these constants wherever it's applicable: !54191 (merged)
• update color codes for new Ultimate and Premium plans: !54685 (merged)
• update rspecs to use new plan constants, do some renaming/refactoring where applicable as well:
  • !54333 (merged)
  • !54680 (merged)
• Billing page: addition of a plan matcher between Gitlab and CustomersDot: !57110 (closed) - closed
• CustomersDot: send premium/ultimate back to GitLab instead of silver/gold: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/2967
• update subscriptions that were created post-EoA but persisted with old plan names: !55625 (merged) to be taken care of as part of this issue.