Adapt to official media type changes in GraphQL over HTTP spec

[bclozel]

As of graphql/graphql-over-http#215, the GraphQL over HTTP spec decided to change the official media type for GraphQL responses over HTTP. This means that:

• clients are expected to send requests as "application/json"
• servers are expected to reply with "application/json" or "application/graphql-response+json", with content negotiation

With this issue, we need to deprecate "application/graphql+json" and introduce support for "application/graphql-response+json".

See spring-projects/spring-framework#29617