Temporarily disable MDNS when MQTT is enabled (#5418)

Leads to a panic

src/mesh/wifi/WiFiAPClient.cpp

@@ -62,7 +62,11 @@ static void onNetworkConnected()
         LOG_INFO("Start WiFi network services");
 
         // start mdns
-        if (!MDNS.begin("Meshtastic")) {
+        if (
+#ifdef ARCH_RP2040
+            !moduleConfig.mqtt.enabled && // MDNS is not supported when MQTT is enabled on ARCH_RP2040
+#endif
+            !MDNS.begin("Meshtastic")) {
             LOG_ERROR("Error setting up MDNS responder!");
         } else {
             LOG_INFO("mDNS Host: Meshtastic.local");