[template] Use C++17 nested namespace syntax (#12346)

esphome/components/template/select/template_select.cpp

@@ -1,8 +1,7 @@
 #include "template_select.h"
 #include "esphome/core/log.h"
 
-namespace esphome {
-namespace template_ {
+namespace esphome::template_ {
 
 static const char *const TAG = "template.select";
 
@@ -63,5 +62,4 @@ void TemplateSelect::dump_config() {
                 YESNO(this->optimistic_), this->option_at(this->initial_option_index_), YESNO(this->restore_value_));
 }
 
-}  // namespace template_
-}  // namespace esphome
+}  // namespace esphome::template_