Update php reqs in BraintreeGraphQl composer.json

Author: pmclain

app/code/Magento/BraintreeGraphQl/composer.json

@@ -3,7 +3,7 @@
     "description": "N/A",
     "type": "magento2-module",
     "require": {
-        "php": "~7.1.3||~7.2.0",
+        "php": "~7.1.3||~7.2.0||~7.3.0",
         "magento/framework": "*",
         "magento/module-braintree": "*",
         "magento/module-store": "*",