audio/musepack: Deprecate and set expiration date to 2024-09-30

No development in over a decade and never got traction, decoding is
supported by FFmpeg however there's no encoding support. For encoding
consider alternative formats such as Opus, AAC, xHE-AAC or legacy
audio formats such as MP3 and Ogg Vorbis.

PR: 281025
Reviewed by: Stefan Ehmann <shoesoft@gmx.net> (maintainer)