Update references to obsolete RFCs

[vytas7]

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.

[Cycloctane]

Should we update the comments and docstrings also?

[vytas7]

Should we update the comments and docstrings also?

@Cycloctane thanks for your interest in this issue!
Docstrings, absolutely, they are a part of the docs.
Comments ideally too, but then they also need to be verified whether the new RFC is fully equivalent.

[vytas7]

(We could also split this issue to address the Rst documents and code docstrings separately. We are accepting partial contributions on this one too.)