sqm-scripts: Change iptables dependency to iptables-nft

sqm-scripts: Change iptables dependency to iptables-nft

There's only one of the shaper scripts (simple.qos) that uses iptables, and
it should be fine with iptables-nft for compatibility with the new default
nft-based firewall. Confusingly, we still need the iptables-mod-ipopt
package to get the DSCP match module; we never used CONNMARK, though, so
drop the iptables-mod-conntrack-extra dependency while we're at it.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>