treewide: deprecate ip-up.target (#18319)

Systemd upstream provides targets for networking. This also includes a target network-online.target.

In this PR I remove / replace most occurrences since some of them were even wrong and could delay startup.

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

@@ -142,7 +142,7 @@ 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 ip-up.target
+                  ${config.systemd.package}/bin/systemctl start network-online.target
                 '';
             preStop = flip concatMapStrings (ips) (ip:
                 let