Switch RFC 7231 to RFC 9110

[philsturgeon]

Similar to #241, RFC 7231 has been obsoleted by RFC 9110 & 9112.

The requestBody is fully supported in HTTP methods where the HTTP 1.1 specification [RFC7231] Section 4.3.1 has explicitly defined semantics for request bodies.

RFC 9110 refers to a HTTP request body as "content" instead of "payload" or "payload body", and mentions in the equivilent method semantics sections:

https://www.rfc-editor.org/rfc/rfc9110#section-9.3.1-6

Although request message framing is independent of the method used, content received in a GET request has no generally defined semantics, cannot alter the meaning or target of the request, and might lead some implementations to reject the request and close the connection because of its potential as a request smuggling attack (Section 11.2 of [HTTP/1.1]). A client SHOULD NOT generate content in a GET request unless it is made directly to an origin server that has previously indicated, in or out of band, that such a request has a purpose and will be adequately supported. An origin server SHOULD NOT rely on private agreements to receive content, since participants in HTTP communication are often unaware of intermediaries along the request chain.

That's still a "its weird probably dont do it" but its a bit more clearly specified.

I'd make a PR but im honestly not sure what's the best wording to change it to.