[select] Modernize namespace declarations to C++17 syntax (#12007)

esphome/components/select/select.cpp

@@ -4,8 +4,7 @@
 #include "esphome/core/log.h"
 #include <cstring>
 
-namespace esphome {
-namespace select {
+namespace esphome::select {
 
 static const char *const TAG = "select";
 
@@ -86,5 +85,4 @@ optional<std::string> Select::at(size_t index) const {
 
 const char *Select::option_at(size_t index) const { return traits.get_options().at(index); }
 
-}  // namespace select
-}  // namespace esphome
+}  // namespace esphome::select