chore(deps): require graphql-php v15

Author: simPod

composer.json

@@ -29,7 +29,7 @@
     "guzzlehttp/promises": "^1.5.0",
     "phpunit/phpunit": "^9.5.10",
     "react/promise": "^2.8.0",
-    "webonyx/graphql-php": "^14.0"
+    "webonyx/graphql-php": "^15.0"
   },
   "suggest": {
     "guzzlehttp/promises": "To use with Guzzle promise",