Add messaging about user overage process to purchase flow

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR tackles the first part of #350379 (closed): introducing a permanent alert to notify the customers about the QSR process. Read more here.

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How to set up and validate locally

1. Run this command:

   export CUSTOMER_PORTAL_URL=https://customers.staging.gitlab.com && gdk restart

2. Make sure to have a group setup
3. Visit the plan purchase flow appening /-/subscriptions/new?plan_id=2c92c0f876e0f4cc0176e176a08f1b70 to your gdk URL

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.