chore: add changelog for allow_subadmins setting

Author: phil-davis

changelog/unreleased/41634

@@ -0,0 +1,8 @@
+Security: disable group-admin feature by default behind allow_subadmins
+
+Disable the subadmin (group-admin) feature by default behind a new
+allow_subadmins system config,as a security risk-mitigation.
+The feature's code path has known security shortcomings; deployments
+that rely on it can opt back in with 'allow_subadmins' => true in config.php.
+
+https://github.com/owncloud/core/pull/41634