net-p2p/libbt: Deprecate and set expiration date to 2024-03-31

Last release in 2007 and no active development, lacks functionality
found in modern implementations.
Redirect users to net-p2p/libtorrent-rasterbar or
net-p2p/libtorrent-rasterbar2

PR: 276980
Approved by: portmgr (maintainer timeout, 2+ weeks)