this and similar issues are the main reason we switched to a non-Grub UEFI bootloader setup with 6.0 - the Grub ZFS implementation is nearly unmaintained and severely behind the actual ZFS on Linux code base (it is basically a read-only parser for the on-disk ZFS data structures from a couple of years ago), with some as-yet-unfixed but hard/impossible to reproduce bugs that can lead to unbootable pools.. all the writes from the dist-upgrade probably made some on-disk structure line up in exactly one of those ways that Grub chokes on it. you can try to randomly copy/move/.. the kernel and initrd files in /boot around in the hopes of them being rewritten in a way that Grub "likes" again..
but the sensible way forward if you still have free space (or even ESP partitions, like if the server was setup with 5.4) on your vdev disks is to use "pve-efiboot-tool" to opt into the new bootloader setup. if that is not an option, you likely need to setup some sort of extra boot device that is not on ZFS, or redo the new bootloader setup by backup - reinstall - restore. we tried hard to investigate and fix these issues within Grub (I lost track of the number of hours I spent digging through Grub debug output via serial console sometime last year, and can personally attest that there are many many more fun ways to spend your time
), but in the end it is sometimes easier to cut your losses and start from scratch. as an intermediate solution / quick fix to get your system booted again, consider moving or copying your /boot partition to some external medium like a high-quality usb disk, or a spare disk if you have one.