Things discovered in Ubuntu 21.04

[ripleymj]

Supporting Ubuntu 21.04 is not a goal or a project, but here's a few things I've found so far that I'll document here in case they become useful later:

• Removing linux*oem in the OEM role fails
• The grub syntax has changed in Packer
• Thonny has been removed due to a conflict with Python 3.9 (https://launchpad.net/ubuntu/+source/thonny/+changelog) and Debian #972723
• Preseed requires a value for ubiquity/use_nonfree (Focal may require ubiquity/prepare_automatic in a future update, Ubuntu bug 1895351)

[ripleymj]

Seems like we're doing fine on 22.04 without anything specific here.