build: bump orchestra/testbench to >=6.15

Author: JaZo

composer.json

@@ -11,7 +11,7 @@
         "friendsofphp/php-cs-fixer": "^3.0",
         "guzzlehttp/guzzle": "^7.3",
         "guzzlehttp/psr7": "^2.1",
-        "orchestra/testbench": "^6.0|^7.0|^8.0",
+        "orchestra/testbench": "^6.15|^7.0|^8.0",
         "phpunit/phpunit": "^9.5"
     },
     "autoload": {