Merge pull request #38466 from nextcloud/fix/read-only-system-addres-book-acls-stable20

[stable20] fix(carddav): Mark system address book as read-only

apps/dav/lib/CardDAV/SystemAddressbook.php

@@ -50,4 +50,13 @@ class SystemAddressbook extends AddressBook {
 
 		return parent::getChildren();
 	}
+
+	public function getACL() {
+		return array_filter(parent::getACL(), function($acl) {
+			if (in_array($acl['privilege'], ['{DAV:}write', '{DAV:}all'], true)) {
+				return false;
+			}
+			return true;
+		});
+	}
 }