treewide: fix fallout of ip-up deprecation

See #18319 for details. Starting network-online.target manually does not
work as it hangs indefinitely.

Additionally, don't treat avahi and dhcpcd special and sync their systemd units
with the respective upstream suggestion.

nixos/modules/tasks/network-interfaces-scripted.nix

@@ -142,7 +142,6 @@ in
                     # (Flushing this interface may have removed it.)
                     ${config.systemd.package}/bin/systemctl try-restart --no-block network-setup.service
                   fi
-                  ${config.systemd.package}/bin/systemctl start network-online.target
                 '';
             preStop = flip concatMapStrings (ips) (ip:
                 let