* nixos/nftables: Allow use with iptables
Since iptables and nftables do not actually conflict with each other, there's no real reason to artificially prevent people from combining them.
In fact, this practice is known to cause issues like #88643, which is fixed by this commit.
services.networking.firewall might have existed during import of this
module in 2016, but it is unknown as of today.
Point to the proper boolean knob to avoid confusion.
reason:
- We currently have an open discussion regarding a more modular
firewall (https://github.com/NixOS/nixpkgs/issues/23181) and
leaving null makes future extension easier.
- the current default might not cover all use cases (different ssh port)
and might break setups, if applied blindly
