magento/graphql-ce#540: Replace deprecated fixture

1. Use POST for mutation query instead of GET

Author: atwixfirster

dev/tests/api-functional/testsuite/Magento/GraphQl/Ups/SetUpsShippingMethodsOnCartTest.php

@@ -567,6 +567,6 @@ private function sendRequestWithToken(string $query): array
         $customerToken = $this->customerTokenService->createCustomerAccessToken('customer@example.com', 'password');
         $headerMap = ['Authorization' => 'Bearer ' . $customerToken];
 
-        return $this->graphQlQuery($query, [], '', $headerMap);
+        return $this->graphQlMutation($query, [], '', $headerMap);
     }
 }