net-p2p/phex: Deprecate and set expiration date to 2025-04-30

Abandonware, last update upstream in 2009.
Point users to net-p2p/gtk-gnutella (unmaintained) but is still active.

Reference:
https://github.com/gtk-gnutella/gtk-gnutella

PR: 285231
Reviewed by: jhale (maintainer)