[BS5] add whats new modal after update

data/web/templates/admin.twig

@@ -70,6 +70,6 @@ var admin_username = '{{ mailcow_cc_username }}';
 var csrf_token = '{{ csrf_token }}';
 var pagination_size = '{{ pagination_size }}';
 var log_pagination_size = '{{ log_pagination_size }}';
-
+var last_login = '{{ last_login }}';
 </script>
 {% endblock %}