Recreate tree based on bullseye rather than buster package

My original attempt to importing user-mode-linux was to take the buster package and update for 5.4. I'd missed that the package in bullseye had already been updated to 5.4. As we are using the 5.4 kernel in Apertis (rather than the 4.19 version found in buster) and to avoid potentially diverging from upstream, pull in the bullseye package instead of the buster one.