[SaaS] Add messaging about user overage process to Renewal flow

Problem to solve

SaaS customers are being surprised and taken off-guard by Quarterly Subscription reconciliations. Because self-service SaaS customers have not historically been charged true-ups, the QSR process and the fact that they have to pay for overages is totally new. See tickets from customers here.

Intended users

This issue is just for SaaS customers.

• Group Owners who are renewing a subscription through our self-service renewal flow
• See exactly which use cases we're solving for in the Epic description.

Proposal

When a user is renewing GitLab, make sure that they are aware of the overage process. Show a message to all customers using the self-service renewal flow that notifies them about the overage process.

Purchase Design here, could we reuse for renewal flow?

Link for How does billing work?: https://docs.gitlab.com/ee/subscriptions/quarterly_reconciliation.html (shared here)