Update the $decoder property to be noted as deprecated

Author: dmanners

lib/internal/Magento/Framework/Webapi/Rest/Request/Deserializer/Json.php

@@ -12,7 +12,10 @@
 
 class Json implements \Magento\Framework\Webapi\Rest\Request\DeserializerInterface
 {
-    /** @var \Magento\Framework\Json\Decoder */
+    /**
+     * @var \Magento\Framework\Json\Decoder
+     * @deprecated
+     */
     protected $decoder;
 
     /**