In include/seastar/http/reply.hh and src/http/reply.cc we have a partial list of HTTP status codes, to which once in a while we need to add a missing status code when a new application notices it's missing.
Let's make this list complete once and for all. The complete list for classic HTTP can be found in https://tools.ietf.org/html/rfc7231#section-8.2.3 and an even more complete list including various HTTP extensions is https://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml

Another newer standard list (mentioning some codes not in RFC231, including 308) is: https://datatracker.ietf.org/doc/html/rfc9110