chore: add SPDX license headers to all repository files

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: szaimen <42591237+szaimen@users.noreply.github.com>

php/domain-validator.php

@@ -1,6 +1,10 @@
 <?php
 declare(strict_types=1);
 
+
+// SPDX-FileCopyrightText: 2023 Nextcloud GmbH and Nextcloud contributors
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
 $domain = '';
 if (isset($_GET['domain']) && is_string($_GET['domain'])) {
     $domain = $_GET['domain'];