Comment 2 for bug 1988299

I'm in a similar situation, running 20.04 with FDE (luks) wanting to upgrade to 22.04. I'm also getting the error message that /boot has not enough free space (needing 575MB free space it says for me), aborting the upgrade. If I try to purge the older of the two currently installed kernels (linux-image-5.15.0-43-generic and linux-image-5.15.0-46-generic are installed) via synaptic to free up some space on /boot, automatically another (second) kernel image is marked to be installed ("linux-image-unsigned-5.15.0-43-generic"), actually taking up more space than freeing... :-/

$ df -h /boot/
Filesystem Size Used Avail Use% Mounted on
/dev/sda5 703M 275M 377M 43% /boot