Issue description

Currently we are supporting setting a global default gateway for each NixOS system. This is problematic in a few ways.
What do we do if we have multiple interfaces? When, where and how it will be added? Will it always be added to all interfaces or just the first or some magic solution?
Should be only support that if we also have a static address and add it to the interface where the route fits best? Is it feasible to expose all those knobs?

The topic came up (again) while I did work on the Systemd v242 migration (#61321). We have to deploy a hack to keep the semantics the same while still not creating multiple (invalid) default routes. @fpletz pointed out #61321 (review) that want to deprecate that we should deprecate the interface.

This issue should serve as a place to track and potentially discuss further steps.