www/subsonic-standalone: Deprecate and set expiration date to 2024-09-30

Last release in 2019 and no longer maintained upstream.
There's a fork being maintained here called Airsonic-Advanced if someone
wants to pick it up.
https://github.com/airsonic-advanced/airsonic-advanced

References:
http://forum.subsonic.org/forum/viewtopic.php?f=4&t=16604#p71128

PR: 281047
Approved by: portmgr (maintainer timeout, 2+ weeks)