fix lowest supported Serializer version

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

Author: xabbuh

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",