remove constructor argument type-hint

The child class Message does not call the parent constructor which means
that the $message property will never be initialized when using CPP.

Author: xabbuh

src/Symfony/Component/Mime/RawMessage.php

@@ -24,7 +24,7 @@ class RawMessage
      * @param iterable<string>|string|resource $message
      */
     public function __construct(
-        private mixed $message,
+        private $message,
     ) {
     }