shells/rssh: Mark as deprecated and set to expire

Upstream has officially abandoned the port some time ago and the software
was maintained then for a while by the Debian project.

But even Debian will now pull the plug and rssh won't be available in their
next stable release. Considering these facts it makes sense to let this port
expire towards the end of Q2.

PR: 235121
Approved by: miwi (mentor), maintainer
Differential Revision: https://reviews.freebsd.org/D19503