Fix various compatibility issues with PHP 8

libs/swiftmailer/classes/Swift/SignedMessage.php

@@ -0,0 +1,23 @@
+<?php
+
+/*
+ * This file is part of SwiftMailer.
+ * (c) 2004-2009 Chris Corbyn
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+/**
+ * Signed Message, message that can be signed using a signer.
+ *
+ * This class is only kept for compatibility
+ *
+ *
+ * @author     Xavier De Cock <xdecock@gmail.com>
+ *
+ * @deprecated
+ */
+class Swift_SignedMessage extends Swift_Message
+{
+}