And in any case, not even remotely related to Virtualmin.
Check ‘uname -a’ to find out which kernel version you are currently running.
Then you can proceed by running ‘apt remove linux-image-(insert the older versions here)’.
Alternatively you can also run ‘apt autoremove’ but be careful to see what is actually going to be removed so that you don’t remove anything important by accident.
It’s not completely irrelevant, but technically this would have been a case for ubuntu forums or the likes.
Webmin does not do any updates, your package manager does.
This is also where you should fix this, as I mentioned above.
Do let us know how things turn out