Switch to systemctl instead of init.d when Monit has to restart SSH

Using systemctl start/stop ssh is prefered instead of /etc/init.d/ssh start/stop because ssh has its SystemD service. Using systemctl method is compatible even with Buster and it will becomes a moment when init.d services won't be supported at all in Debian. Exception is for distros which deleted Systemd but these are not supported by ISPConfig. So modifying setup_monit tool to use systemctl looks good also for future compatibility, without breaking some old Debian versions.