The leading to the unsupported message was updated.
All you need to to is run “sudo update-grub”. It is mentioned in https://github.com/home-assistant/supervisor/issues/3715#issuecomment-1176312079.
Here is the link to the solution:
Updated to latest supervisor and despite mentioned changes still get "unsupported installation" - #6 by CentralCommand.
I hope your setup will run smooth after the update-grub command ran.