drop support for Symfony < 4.3

Author: Mathieu Lemoine

composer.json

@@ -10,15 +10,15 @@
         "enqueue/enqueue": "^0.9",
         "queue-interop/amqp-interop": "^0.8",
         "queue-interop/queue-interop": "^0.7|^0.8",
-        "symfony/config": "^3.4|^4|^5"
+        "symfony/config": "^4.3|^5"
     },
     "require-dev": {
         "phpunit/phpunit": "~7.5",
         "enqueue/test": "0.9.x-dev",
         "enqueue/amqp-ext": "0.9.x-dev",
         "enqueue/fs": "0.9.x-dev",
         "enqueue/null": "0.9.x-dev",
-        "symfony/yaml": "^3.4|^4|^5"
+        "symfony/yaml": "^4.3|^5"
     },
     "support": {
         "email": "[email protected]",