Payment Buttons block - add minimum amount option

### Impacted plugin

Jetpack

### What

In Payment Buttoms block - add minimum amount option when "Enable customers to pick their own amount" is seta

### How

When setting up a "Payment Buttoms" feature there is an option to "Add a new payment plan". A payment plan has an option to "Enable customer to pick their own amount". At present when that option is set, the customer can enter any amount. An automatic minimum of $0.50 is enforced. This should be made variable to allow a higher minimum.

The issue concerns the Jetpack Payment Buttons block and its payment-plan setup, but names no files or tests. Start by locating the block's handling of "Enable customer to pick their own amount" and trace where the automatic $0.50 minimum is enforced. Done means a configurable higher minimum can be set and is enforced for customer-entered amounts.

Written by the indexing model from the issue text.