`isOverflowVisible` not required on `AlertModal`

[brian-smith-tcril]

#3503 added this as a required prop for ModalDialog, but it seems MFEs have gotten away with continuing to not specify the value on AlertModals.

I noticed this because of a snapshot mismatch on openedx/frontend-app-account#1266.

[brian-smith-tcril]

• AlertModal should have isOverflowVisible as a required (with not default) prop because it's using ModalDialog under the hood
  • Need to do more investigation as to why that is not the case
    • Possibly just an issue with snapshot testing (maybe mocking components?)