This commit removes iptables backend support and leaves only the
netfilter backend support. This means that:
-  iptables and nftables firewall based systems (firewall3 and firewall
4) are supported trough the netfilter instance mode
-  the iptables/xtables mode support is disabled

For more information on the modes and how to use the new netfilter
instance checkout https://www.jool.mx/en/intro-jool.html

This move is made out of the commit upstream that sets firewall4 as the
default for new default buils and based on the conversation in #16818
and was decided that the netfilter interface is the priority since
iptables support will be dropped in the foreseeable future.

While at it update the templates provided.

Signed-off-by: Tiago Gaspar <tiagogaspar8@gmail.com>