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

Abandoned upstream, lacks support for common functionality such as
Magnet links, DHT, PEX and HTTPS.
Redirect users to net-p2p/qbittorrent or net-p2p/transmission

PR: 276977
Approved by: portmgr (maintainer timeout, 2+ weeks)