fix lowest supported Serializer version

The `AbstractObjectNormalizer::setMaxDepthHandler()` method does not
exist before `symfony/serializer` 4.1.

src/Symfony/Bundle/FrameworkBundle/composer.json

@@ -67,7 +67,7 @@
         "symfony/console": "<3.4",
         "symfony/form": "<3.4",
         "symfony/property-info": "<3.4",
-        "symfony/serializer": "<3.4",
+        "symfony/serializer": "<4.1",
         "symfony/stopwatch": "<3.4",
         "symfony/translation": "<3.4",
         "symfony/validator": "<4.1",

src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

@@ -214,7 +214,7 @@ abstract class AbstractObjectNormalizer extends AbstractNormalizer
     abstract protected function getAttributeValue($object, $attribute, $format = null, array $context = array());
 
     /**
-     * Sets an handler function that will be called when the max depth is reached.
+     * Sets a handler function that will be called when the max depth is reached.
      */
     public function setMaxDepthHandler(?callable $handler): void
     {