Deprecate usermod mqtt_switch. (#2499)

usermods/mqtt_switch_v2/README.md

@@ -1,3 +1,7 @@
+# DEPRECATION NOTICE
+This usermod is deprecated and no longer maintained. It will be removed in a future WLED release. Please use usermod multi_relay which has more features.
+
+
 # MQTT controllable switches
 This usermod allows controlling switches (e.g. relays) via MQTT.
 

usermods/mqtt_switch_v2/usermod_mqtt_switch.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#warning "This usermod is deprecated and no longer maintained. It will be removed in a future WLED release. Please use usermod multi_relay which has more features."
+
 #include "wled.h"
 #ifndef WLED_ENABLE_MQTT
 #error "This user mod requires MQTT to be enabled."