Upgrade isobuilders to Bullseye

Our Jenkins isobuilders currently run Buster. By the time Buster is not supported anymore (~2022, with no info for LTS yet) we'll need to update them to Bullseye.

Also, a more recent virtualization stack (libvirt, QEMU, Linux kernel) may provide performance improvements, that would be most welcome (#16960 (closed)). If this upgrade is cheap, it might be worth applying it earlier rather than later.

Let's figure out what's needed to make it happen!

Related issues

Blocks tails#17976 (closed)

EDIT: this issue was created when builders were running Stretch, so the above description has been edited to account for their upgrade to Buster. Have this in mind when reading the issue history.

Edited Mar 03, 2021 by Zen Fu

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information