Deprecate support of the application/graphql+json media type #915

In graphql/graphql-over-http#161 this was added to the draft spec:

A server MUST support requests from clients with HTTP header Content-Type: application/graphql+json indicating that the body of the request is a JSON document with a GraphQL request.

GraphQL.NET Server v7 supports both the application/json and application/graphql+json media types for requests.

This media type has been removed in graphql/graphql-over-http#175 where we have this:

The following are the officially recognized GraphQL media types to designate using the JSON encoding for GraphQL requests:

Name	Description
application/json	Standard type for GraphQL JSON requests

Although no change is required, we may wish to note in the source that support was removed for this media type within the current draft spec.