Add back Plugin class for backwards compatibility

Author: ADmad

src/Plugin.php

@@ -0,0 +1,23 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ * @link          https://cakephp.org CakePHP(tm) Project
+ * @since         1.0.2
+ * @license       https://www.opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Authentication;
+
+/**
+ * @deprecated 3.3.4 Use AuthenticationPlugin instead
+ */
+class Plugin extends AuthenticationPlugin
+{
+}