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

esphome/components/ble_client/sensor/ble_sensor.cpp

@@ -6,8 +6,7 @@
 
 #ifdef USE_ESP32
 
-namespace esphome {
-namespace ble_client {
+namespace esphome::ble_client {
 
 static const char *const TAG = "ble_sensor";
 
@@ -147,6 +146,5 @@ void BLESensor::update() {
   }
 }
 
-}  // namespace ble_client
-}  // namespace esphome
+}  // namespace esphome::ble_client
 #endif