audio/flake: Deprecate and set expiration date to 2021-12-31

Upstream lists software as beta, no new release since 2007
Users also reports bugs when libsndfile isn't utilized
Users are recommended to use audio/ffmpeg or audio/flac instead

Reference: https://hydrogenaud.io/index.php?topic=71984.0

PR: 258638
Approved by: tcberner and lbartoletti (mentors)
Differential Revision: https://reviews.freebsd.org/D32038