Add support for ffmpeg 5.0

Rebased, slightly improved version of !45 (closed) . As noted on comment !45 (comment 637210) , the implementation follows the existing principle in the file src/musicbrainz/MusicDNSAudioDecoder.cpp dating back 2012, that is, a separate function implementation for different ffmpeg versions.

Compiled and tested very superficially. Clicking the MusicBrains button in song details window doesn't seem to do anything. I'm not quite sure what the MusicBrains is supposed to do (never used it) and I have no idea if it works at the moment with some ffmpeg version / when was the last time it has worked with some, but at least nothing seems to crash. Ok, I wasn't just patient enough, clicking it does something if one waits a moment. However, I'm not quite sure if that comes from ffmpeg or if there's some different ffmpeg-related MusicBrainz functionality, too. (Was it the apparently-since-some-time-defunct MusicDNS system?)

Creates a fine-looking request with a fine-looking fingerprint (except for vorbises due to an apparent ffmpeg bug), but the MusicDNS service is not there any more. But from the perspective of this code, everything seems to be technically correct. These changes also make the opensuse pipeline succeed, so merging this should help catching problems elsewhere.