[Unify purchase flows] SaaS/Self-Managed Premium and Ultimate
## Objective
See the objective outlined in the parent epic https://gitlab.com/groups/gitlab-org/-/epics/12199.
## Purpose
This epic houses the work involved to migrate the existing SaaS/Self-Managed Premium/Ultimate purchase flows to the unified version.
## Issue board
[Issue board](https://gitlab.com/groups/gitlab-org/-/boards/9485135?label_name%5B%5D=Unify%20purchase%20flows%3A%3APremium%2FUltimate)
epic