Fix MQTT Light with RGB and Brightness (#15053)

* Fix MQTT Light with RGB and Brightness

When an MQTT light is given an RGB and Brightness topic, the RGB
is scaled by the brightness *as well* as the brightness being set

This causes 255,0,0 at 50% brightness to be sent as 127,0,0 at 50%
brightness, which ends up as 63,0,0 after the RGB bulb has applied
its brightness scaling.

Fixes the same issue in mqtt, mqtt-json and mqtt-template.

Related Issue: #13725

* Add comment to mqtt_json as well

tests/components/light/test_mqtt_json.py

@@ -381,8 +381,8 @@ class TestLightMQTTJSON(unittest.TestCase):
         self.assertEqual(50, message_json["brightness"])
         self.assertEqual({
             'r': 0,
-            'g': 50,
-            'b': 4,
+            'g': 255,
+            'b': 21,
         }, message_json["color"])
         self.assertEqual("ON", message_json["state"])