Per the adjusted FFmpeg pinning advice, packages that work on the
default version should use the unversioned variants to ease the
migration to future versions and reduce the number of packages that
end up referencing old versions.
I have left HandBrake pinned as it builds a custom patched FFmpeg.
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.
