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"
+      }
    }
 }

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "c1d313295fa1e219082cb159d7ba6c63",
+    "content-hash": "0acbc64f870a4758034898ef4f8a824d",
     "packages": [
         {
             "name": "adhocore/json-comment",
@@ -1139,16 +1139,16 @@
         },
         {
             "name": "phpunit/php-code-coverage",
-            "version": "9.1.6",
+            "version": "9.1.7",
             "source": {
                 "type": "git",
                 "url": "https://github.com/sebastianbergmann/php-code-coverage.git",
-                "reference": "d25b24b1cd14772bde4d75daeb393dc17db9f6e6"
+                "reference": "2ef92bec3186a827faf7362ff92ae4e8ec2e49d2"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-code-coverage/zipball/d25b24b1cd14772bde4d75daeb393dc17db9f6e6",
-                "reference": "d25b24b1cd14772bde4d75daeb393dc17db9f6e6",
+                "url": "https://api.github.com/repos/sebastianbergmann/php-code-coverage/zipball/2ef92bec3186a827faf7362ff92ae4e8ec2e49d2",
+                "reference": "2ef92bec3186a827faf7362ff92ae4e8ec2e49d2",
                 "shasum": ""
             },
             "require": {
@@ -1208,7 +1208,7 @@
                     "type": "github"
                 }
             ],
-            "time": "2020-08-31T06:31:46+00:00"
+            "time": "2020-09-03T07:09:19+00:00"
         },
         {
             "name": "phpunit/php-file-iterator",
@@ -2633,5 +2633,8 @@
         "ext-zlib": ">=1.2.11"
     },
     "platform-dev": [],
+    "platform-overrides": {
+        "php": "7.3.0"
+    },
     "plugin-api-version": "1.1.0"
 }