Merge pull request #26151 from nextcloud/backport/26137/stable21

[stable21] dont allow creating users with __groupfolders as uid

lib/private/User/Manager.php

@@ -681,6 +681,7 @@ class Manager extends PublicEmitter implements IUserManager {
 		if (\in_array($uid, [
 			'.htaccess',
 			'files_external',
+			'__groupfolders',
 			'.ocdata',
 			'owncloud.log',
 			'nextcloud.log',