debian: bump compat to 12 and don't restart container.slice

debian: bump compat to 12 and don't restart container.slice

since compat 10 the restart is default, as I want to use
'dh_installsystemd' (vs 'dh_systemd_start') I need at least compat
level 11, so go for the now recommended compat level 12.

diffoscope tells me that the main change us the wanted:

./postinst
> @@ -1,10 +1,15 @@
>  #!/bin/sh
>  set -e
> -# Automatically added by dh_systemd_start/12.1.1
> +# Automatically added by dh_installsystemd/12.1.1
>  if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
>   if [ -d /run/systemd/system ]; then
>           systemctl --system daemon-reload >/dev/null || true
> -         if [ -n "$2" ]; then
> -                 _dh_action=restart
> -         else
> -                 _dh_action=start
> -         fi
> -         deb-systemd-invoke $_dh_action 'system-pve\x2dcontainer.slice' >/dev/null || true
> +         deb-systemd-invoke start 'system-pve\x2dcontainer.slice' >/dev/null || true
>   fi
>  fi
>  # End automatically added section

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>