Restrict root object identification to ROOT_QUERY.

[benjamn]

Although "Mutation" and "Subscription" are technically the default names for the root mutation and subscription types, there should never be any need to refer to those objects, read from them, or write to them, outside the normal process of sending and receiving mutations and subscriptions.

If you think those use cases have any value, #6911 will quickly convince you that the downsides of monopolizing types like Subscription and Mutation (e.g. assuming they are always singletons) dramatically outweigh the benefits.

In fact, it's not even possible to request the __typename field for the root Subscription object, according to the GraphQL specification: https://spec.graphql.org/June2018/#sec-Single-root-field

In particular, whereas

query { __typename }

can be used to find out the __typename of the root query object (which might be something other than "Query" in rare situations), the following operation is illegal, because a subscription must have only a single root field:

subscription MessagesSub {
  __typename
  messages {
    source
    payload
  }
}

This means the only way to find out that the root Subscription type has a __typename other than "Subscription" would be through an introspection query, which is typically disallowed in production.