Update references to obsolete RFCs

Our documentation is replete with references to now-obsolete RFCs such as RFC 7231, RFC 7238, etc. Many of these references can be replaced by links to RFC 9110 (HTTP Semantics), but please do inspect each of them on a case by case basis, and map the respective section numbers, if needed.

If the specification itself has changed, and we need to update Falcon's behaviour, please file new issues.