Change the deprecated method to a getter method.

src/ExistingMessagesConsumer.php

@@ -38,7 +38,7 @@ public function consume(callable $callback): void
             false,
             function (AMQPMessage $amqpMessage) use ($callback): void {
                 try {
-                    $message = $this->serializer->unserialize($amqpMessage->body);
+                    $message = $this->serializer->unserialize($amqpMessage->getBody());
                     if ($this->messageConsumed = $callback($message)) {
                         $this->channel->basic_ack($amqpMessage->getDeliveryTag());
                     }