Update PHP and PHPUnit across all packages; fix ext-rdkafka requirement

Author: andrewmy

composer.json

@@ -6,14 +6,14 @@
     "homepage": "https://enqueue.forma-pro.com/",
     "license": "MIT",
     "require": {
-        "php": "^7.1.3",
+        "php": "^7.3",
         "queue-interop/queue-interop": "^0.8",
         "enqueue/dsn": "^0.10",
         "symfony/filesystem": "^4.3|^5",
         "makasim/temp-file": "^0.2@stable"
     },
     "require-dev": {
-        "phpunit/phpunit": "~7.5",
+        "phpunit/phpunit": "^9.5",
         "enqueue/null": "0.10.x-dev",
         "enqueue/test": "0.10.x-dev",
         "queue-interop/queue-spec": "^0.6",