Announced removal of non-standard GraphQL error field

To fully comply with the GraphQL response format, we have moved the code field under the extensions map as shown in this example.

From 15 January 2023, code will no longer be returned as a top-level field of the errors array, but under the extensions map.

As a part of the same initiative oriented towards complying with the GraphQL specification, we also removed support for non-standard variables. This change was announced on 4 October 2022 and took effect on 1 December 2022.