Donations Block: Introduce a cap/limit to donations

### Impacted plugin

Jetpack

### What

A customer inquired about the possibility of putting a "cap" on donations or adding a feature to help people double-check the donation amount.

### How

The idea would be to offer a setting where you can add a max donation you'd like to receive. While this can be somewhat arranged by removing the custom amount field, the customer also claims that this could avoid situations where someone donates a large sum, demands a refund from their credit card company, and leaves the site owner with the fees

Suggested on 8140040-zen

The issue identifies the Jetpack Donations Block but names no files, tests, or entry points. Start by locating the Donations Block implementation and its existing amount-field settings; done should include an agreed maximum-donation or amount-confirmation behavior that addresses the stated refund concern.

Written by the indexing model from the issue text.