Comment 6 for bug 1969353

In the meantime, you can do this to fix your system:

sudo cp /usr/lib/grub-customizer/grubcfg-proxy /etc/grub.d/bin/grubcfg_proxy

EDIT: see https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1969353/comments/14 for a new workaround since the package has been completely removed from the archive