I'm working on deprecating the top-level options, that configure main.cf
upstream in nixpkgs. With this change we stay ahead of the curve.
The `networks_style` option already defaults to `host` since Postfix 3.0,
so I dropped the setting.
```
$ postconf -d | grep networks_style
mynetworks_style = ${{$compatibility_level} <level {2} ? {subnet} : {host}}
````
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