text: Title: Update `BaseGroupApprovalTaskStateEmailNotifier` to check for `AbstractGroupApprovalTask` instances
  Body: #11653 added `AbstractGroupApprovalTask` and `GroupApprovalTask` inherits from it.
  
  howevere the `BaseGroupApprovalTaskStateEmailNotifier` still checks `GroupApprovalTask` - https://github.com/wagtail/wagtail/blob/6affa04d320f50b6a3babd8afc1a590e02e84a5d/wagtail/admin/mail.py#L368-L373
  
  This means any custom group tasks derived from `AbstractGroupApprovalTask`s require adding custom notifiers and wiring in a signal handler
  