Add isc-dhcp-server to example debootstrap script

Debian 13+ no longer include isc-dhcp-server (or any DHCP client it
seems) in their default minimal stack. This can cause cloud-init to fail
early and render newly provsiioned VMs unbootable. We add it to the
example here for posterity.

api-daemon/provisioner/examples/script/3-debootstrap.py

@@ -485,6 +485,7 @@ class VMBuilderScript(VMBuilder):
                 "acpid",
                 "acpi-support-base",
                 "wget",
+                "isc-dhcp-server",  # required for cloud-init on Debian 13+ where this is no longer standard
             ]
 
         # We need to know our root disk for later GRUB-ing