www/mod_proxy_xml: Mark BROKEN, deprecate and set expiration date to 2022-03-31

Abandoned upstream, last release in 2004 and reported broken

Reference: https://stackoverflow.com/questions/11956514/how-can-i-rewrite-urls-in-xml-with-apache-2-4

Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D32398