sudo apt autoremove
If the above does not remove old ones lets first check the current kernel thats being used
uname -r
Lists the current used kernel version. Lets remove the earlier versions of the kernel upgrades
if the current version is 15.11.8.* all prior versions can be removed
sudo rm /boot/*-5.11.18-{10,9}-*
sudo dpkg --configure -a
sudo apt install -f
sudo update-grub
sudo apt autoremove
sudo apt upgrade