Update iptables systemd unit to use new version of iptables-restore

Since Debian Buster, old iptables/xtables (legacy) and new nftables frameworks can be used in parallel.
We chose to use the legacy framework (see !399 (merged)), but since the introduction of Docker in the SDK (767b7717), which set its specific rules using nftables, this breaks the connman tethering as there are two sets running.

iptables-nft-restore is able to apply iptables rules to nftables.
/usr/sbin/iptables-restore is a symlink to /etc/alternatives/iptables-restore which in turn is a symlink to /usr/sbin/iptables-nft-restore. Same for /usr/sbin/ip6tables-restore.

Replacing iptables-legacy-restore by iptables-restore allows to use the nftables only.

https://phabricator.apertis.org/T8361

Signed-off-by: Frédéric Danis frederic.danis@collabora.com