multimedia/ffmpeg: update to 6.1

• HARFBUZZ (like ASS option) is enabled to keep -vf drawtext
• Rebuild mpv to expose --hwdec=vulkan

Changes: https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/n6.1:/Changelog
PR: 272575
Reported by: Repology
Exp-run by: antoine