How to get more space on /boot? (extra kernels have been removed)

i'm on ubuntu 21.04 trying to update to 21.10, and getting the usual error

The upgrade has aborted. The upgrade needs a total of 287 M free space on disk '/boot'. Please free at least an additional 31,6 M of disk space on '/boot'. You can remove old kernels using 'sudo apt autoremove' and you could also set COMPRESS=xz in /etc/initramfs-tools/initramfs.conf to reduce the size of your initramfs.

i usually fix this error by following obvious instructions like those here, but not this time:

$ ls /boot
config-5.11.0-22-generic lost+found vmlinuz
config-5.11.0-25-generic memtest86+.bin vmlinuz-5.11.0-22-generic
grub memtest86+.elf vmlinuz-5.11.0-25-generic
initrd.img memtest86+_multiboot.bin vmlinuz.old
initrd.img-5.11.0-22-generic System.map-5.11.0-22-generic
initrd.img.old System.map-5.11.0-25-generic

$ dpkg -l linux-image-\* | grep ^ii
ii linux-image-5.11.0-22-generic 5.11.0-22.23 amd64 Signed kernel image generic
ii linux-image-5.11.0-25-generic 5.11.0-25.27 amd64 Signed kernel image generic

it doesn't look like i have too many kernels. (5.11.0-22 is what i'm running, i think 5.11.0.25 is what i'm upgrading to?) i'm guessing there may be some issue with LVM and having an encrypted LUKS partition?

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