[light] Modernize namespace declarations to C++17 syntax (#11986)

esphome/components/light/light_output.cpp

@@ -1,12 +1,10 @@
 #include "light_output.h"
 #include "transformers.h"
 
-namespace esphome {
-namespace light {
+namespace esphome::light {
 
 std::unique_ptr<LightTransformer> LightOutput::create_default_transition() {
   return make_unique<LightTransitionTransformer>();
 }
 
-}  // namespace light
-}  // namespace esphome
+}  // namespace esphome::light