[FE] Breakdown: Move the GitLab.com upgrade purchase flow to GitLab.com
## Proposal
Move the GitLab.com upgrade purchase flow to GitLab.com CTA.
### Success Criteria
- The user should remain in GitLab.com from a UI perspective
- The user should not be prompted to log into the Web Store
- The user should not be prompted to enter a namespace for the purchase
### Design
[See design issue](https://gitlab.com/gitlab-org/gitlab/-/issues/333825)
issue