[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)
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