[Bug] Disallow sorting by child fields that are arrays

Bug report

In a subgraph schema like

type Parent {
  id: ID!
  children: [Child!]!
}

we allow a query like parents(orderBy: children__id) { .. } but that ordering doesn't make much sense. When a parent has multiple children, it's not clear which child's id we will use for the ordering. We should simply not support that.

Some information to help us out

• Tick this box if this bug is caused by a regression found in the latest release.
• Tick this box if this bug is specific to the hosted service.
• I have searched the issue tracker to make sure this issue is not a duplicate.

OS information

None

En el issue no se nombra ningún archivo, prueba ni punto de entrada. Empieza rastreando cómo se validan los campos GraphQL orderBy para relaciones anidadas y, después, reproduce el caso de esquema Parent/children; estará terminado cuando se rechace ordenar por un campo de tipo array de un elemento hijo, mientras las relaciones escalares compatibles sigan siendo válidas.