Update ffmpeg to 6.1.2 for a memory leak fix

See
https://forums.bluecherrydvr.com/t/memory-leak-after-update-to-3-1-2-on-ubuntu-22lts/905/6
https://ffmpeg.org//pipermail/ffmpeg-devel/2024-May/326569.html

Suggested-by: Curtis Hall <[email protected]>