The w3.org RFC 2616 page displays an obtrusive "This document has been
superseded" overlay.  In regard to the `Cache-Control` header, RFC 2616
has been superseded by RFC 7234, which, in turn, has been superseded by
RFC 9111.

Therefore, this commit replaces links to RFC 2616 with links to either
MDN or RFC 9111.

(cherry picked from commit 9a82d958)