Deprecate "application/graphql+json" media type after spec changes

[bclozel]

As of graphql/graphql-over-http#215, the new media type considered for GraphQL responses by the HTTP spec has been changed from "application/graphql+json" to "application/graphql-response+json". We should deprecate the former.

As for introducing the new one as a replacement, this might be done in Spring GraphQL directly as it's being used mainly in that project.