Allow usage of ext-rdkafka 6

Author: jdecool

composer.json

@@ -20,7 +20,7 @@
     "ext-amqp": "^1.9.3",
     "ext-gearman": "^2.0",
     "ext-mongodb": "^1.5",
-    "ext-rdkafka": "^4.0|^5.0",
+    "ext-rdkafka": "^4.0|^5.0|^6.0",
 
     "queue-interop/amqp-interop": "^0.8.2",
     "queue-interop/queue-interop": "^0.8.1",