nixos/networkd: rename the networkd dhcpConfig option to dhcpV4Config

This follows upstreams change in documentation. While the `[DHCP]`
section might still work it is undocumented and we should probably not
be using it anymore. Users can just upgrade to the new option without
much hassle.

I had to create a bit of custom module deprecation code since the usual
approach doesn't support wildcards in the path.