root-on-ZFS system unbootable after upgrade to 21.04

[Nick Crabtree]

Several years ago I performed a fresh installation of Ubuntu 18.04 following the instructions at the zfs-on-linux site.
I then recently upgraded to 20.04 and then 20.10 using 'do-release-upgrade'. The upgrades were both successful and the system rebooted fine.
I then yesterday further upgraded to 21.04 and the system no longer reboots.

I don't have any way of copy-pasting the output from the monitor, so here is my attempt at typing in the information from the photo I took of the screen, of what I think are the key lines (with apologies for any typos):

[ 2.139322] ZFS: Loaded module v2.0.2-1ubuntu5, ZFS pool version 5000, ZFS filesystem version 5
initramfs: Parameter root=ZFS=bionic-rpool/ROOT/ubuntu unsupported (only /dev, 0xMAJMIN and UUID= are supported)
initramfs: Will cause kernel panic in 10s...
[ 12.218201] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000100
[ 12.246539] CPU: 1 PID: 1 Comm: init Tainted: P 0 5.11.0.22-generic #23-Ubuntu

What I expected to happen: The upgrade process would leave me with a bootable system
What happened instead: The system will not boot

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/1936304/+editstatus and add the package name in the text box next to the word Package.

[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]