nixos/mosquitto: wait for network-online.target, not network.target

network.target is reached earlier, but with much fewer services
available. DNS is likely to be not functional before
network-online.target, so waiting for that seems better for that reason
alone. the existing backends for network-online.target all seem to do
reasonable things (wait until all links are in *some* stable state), so
we shouldn't lose anything from waiting.

nixos/modules/services/networking/mosquitto.nix

@@ -556,7 +556,7 @@ in
     systemd.services.mosquitto = {
       description = "Mosquitto MQTT Broker Daemon";
       wantedBy = [ "multi-user.target" ];
-      after = [ "network.target" ];
+      after = [ "network-online.target" ];
       serviceConfig = {
         Type = "notify";
         NotifyAccess = "main";