Clear ExternalCustomerId (and possibly set note), when a Drupal user is being deleted

Problem/Motivation

Clear ExternalCustomerId (and possibly set note), when a Drupal user is being deleted.

Otherwise the Webhooks cause errors like this, looking for the user:
Drupal\billwerk_subscriptions\Exception\WebhookException: Subscriber user account for Billwerk Customer ID #1234254234623452345 with ExternalCustomerId #12345 could not be found. No such user exists. Could not update the user subscription. in Drupal\billwerk_subscriptions\EventSubscriber\BillwerkWebhookEventSubscriber->onCustomerChanged() (line 115 of /web/modules/contrib/billwerk_subscriptions/src/EventSubscriber/BillwerkWebhookEventSubscriber.php).

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes