feat: Document autoloader removal in 32

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

Author: come-nc

developer_manual/app_publishing_maintenance/app_upgrade_guide/upgrade_to_32.rst

@@ -215,3 +215,5 @@ Removed APIs
   - ``IServerContainer::getSystemTagManager()``
   - ``IServerContainer::getSystemTagObjectMapper()``
   - ``IServerContainer::getTagManager()``
+
+- The legacy autoloader ``\OC::$loader`` was removed. It should not impact your application. It may impact your tests if they were using it. Please consider including ``tests/autoload.php`` from server instead.