chore: initialize config.platform.php with PHP 8.1 (#100)

Set the `config.platform.php` key in `composer.json` to initialize with
PHP version 8.1. This ensures Composer assumes the specified PHP version
for the project's platform.

Signed-off-by: Valentin Sickert <[email protected]>

Author: Lapotor

composer.json

@@ -61,6 +61,9 @@
         "allow-plugins": {
             "pestphp/pest-plugin": true,
             "php-http/discovery": true
+        },
+        "platform": {
+            "php": "8.1"
         }
     },
     "minimum-stability": "stable",