Spec references RFC7231 which is obsoleted

Here are couple of things I found that might be an issue:

Section [Response](https://github.com/graphql/graphql-over-http/blob/main/spec/GraphQLOverHTTP.md#response)

> A server must comply with RFC7231.

First, it would be nice to provide RFC title along with numeric ID, just as a nice gesture to the reader.
Secondly, this RFC page has a note on top: "Obsoleted by: 9110"; should the spec reference the newer RFC? otherwise some comment is needed.

Note: If changing ref to 9110, then some other links to its subsections should be updated. Ex: link to "HTTP 1.1 Accept" section.