composer: do not install packages with min version higher than 7.3.0

running composer update on 7.4 will generate a lock file using the newest dependencies which work for the current PHP version, which usually isn't desirable for a project like this where developers might be using newer PHP versions than users.

composer.json

@@ -57,5 +57,10 @@
       "psr-4": {
          "pocketmine\\": "tests/phpunit/"
       }
+   },
+   "config": {
+      "platform": {
+         "php": "7.3.0"
+      }
    }
 }