ffmpeg: switch default to ffmpeg_8

  # unversioned aliases to allow for quicker migration to new releases,

  # but can pin one of the versioned variants if they do not work with

  # the current default version.

  ffmpeg = ffmpeg_7;

  ffmpeg-headless = ffmpeg_7-headless;

  ffmpeg-full = ffmpeg_7-full;

  ffmpeg = ffmpeg_8;

  ffmpeg-headless = ffmpeg_8-headless;

  ffmpeg-full = ffmpeg_8-full;

}