net/iaxmodem: Initiate sunset

This port hasn't been updated despite there being four 1.3.x releases
since 2016 (there is nothing in Bugzilla on that either), and it depends
on GCC 9 which has been end-of-lifed upstream nearly two years ago.

Faxes aren't used much any more and apparently there is no need/use for
this port.

Mark deprecated with an expiration date of 2024-05-28. On the way reset
maintainer to ports@.

Approved by: maintainer