[ble_client] Convert to C++17 namespace style (#12176)

esphome/components/ble_client/switch/ble_switch.cpp

@@ -4,8 +4,7 @@
 
 #ifdef USE_ESP32
 
-namespace esphome {
-namespace ble_client {
+namespace esphome::ble_client {
 
 static const char *const TAG = "ble_switch";
 
@@ -31,6 +30,5 @@ void BLEClientSwitch::gattc_event_handler(esp_gattc_cb_event_t event, esp_gatt_i
 
 void BLEClientSwitch::dump_config() { LOG_SWITCH("", "BLE Client Switch", this); }
 
-}  // namespace ble_client
-}  // namespace esphome
+}  // namespace esphome::ble_client
 #endif