* Increase the minimum free disk for automatic selection of the desktop task

  to 3 gb. Needed since the desktop task has grown somewhat, to around 2.5
  gb, and since several hundred megabytes are needed for debs in /var
  (which might be on the same partition).
  2.5 gb free is tested to currently work in a standard single-partition
  layout when installing standard+desktop+laptop, so this leaves some room
  for future bloat, and for localisation and other tasks. Closes: #405129