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

esphome/components/light/light_effect.cpp

@@ -1,8 +1,7 @@
 #include "light_effect.h"
 #include "light_state.h"
 
-namespace esphome {
-namespace light {
+namespace esphome::light {
 
 uint32_t LightEffect::get_index() const {
   if (this->state_ == nullptr) {
@@ -32,5 +31,4 @@ uint32_t LightEffect::get_index_in_parent_() const {
   return 0;  // Not found
 }
 
-}  // namespace light
-}  // namespace esphome
+}  // namespace esphome::light