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

Deprecated and considered obsolete by upstream in 2016 in favour of (lib)opus
Reference: https://wiki.xiph.org/index.php?title=OggCELT&oldid=16389

PR: 258655
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D32050