[revise] fix payment form code snippet

[CBID2]

Originally from @hamishwillee in #26790
The action is to add required="true" to any field that should be required, and make sure there are corresponding aria markup for those same case. The associated example should be updated in tandem, and merged at the same time: https://github.com/mdn/learning-area/blob/main/html/forms/html-form-structure/payment-form.html

[hamishwillee]

Closing. This issue is not needed for tracking. See mdn/content#26790