Kernel Clean sh add systemd bootloader support

[Vizion-HUN]

Some of us using systemd bootloader, so the update-grub does nothing, we must run proxmox-boot-tool refresh in shell.
Could You please add this to the kernel clean script too?
And as I see, the old config, systemmap, etc... files are not removed from /boot folder. Not much space, but I like it clean.
I don't know if a cleanup in /boot is necessary to run proxmox-boot-tool refresh.

Thank You for Your work!