ffmpegthumbs is no longer compatibile with ffmpeg post
v20.11.80/v21.03.80 due to dropping a call to av_register_all(). Bump
ffmpeg dependency to match new required version.
After making `ffmpeg` point to the latest `ffmpeg_4`, all packages that
used `ffmpeg` without requiring a specific version now use ffmpeg_3
explicitly so they shouldn't change.
