Disappearing users in system address book for federation

[miaulalala]

Follow up to https://github.com/nextcloud/server/issues/38021

With the new address book exposure, a user can remove themselves from a federated the system address book by setting their VCARD data to a local scope on the required VCARD properties (FN (v3, v4), N(v2, v3)).

This is GDPR compliant, but might cause confusion if admins are federating with another instance and users are missing.

Option 1: allow admins force overrride Option 2: document and leave as is