RFC 9111 obsoletes the Warning header, removing all specification
requirements about it. Likewise, this update changes Squid behaviour in
regards to that header:
1) Squid no longer adds Warning headers to generated or forwarded
messages. Miss responses from servers/peers and hits cached by an
older version of Squid may still have Warning headers.
2) On 304 revalidation, Warning header are treated the same as any
other/generic header. They are added or replaced according to their
presence in the 304 reply. Absent any Warning update by a 304, Squid
may still deliver cached content with old Warning headers.
3) Squid no longer validates received Warning headers. RFC 7234 placed
syntax requirements and limits on how old some Warning values could
be (when dated). Those checks are no longer being performed. The
header value is now treated as an opaque string.
4) Warning header usage and types are no longer tracked in message
statistics available through cache manager.
Updated documentation references to reflect RFC 7234 becoming obsolete.