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
