Deprecate support of the `application/graphql+json` media type

In https://github.com/graphql/graphql-over-http/pull/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 https://github.com/graphql/graphql-over-http/pull/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.