changed from deprecated flush() to clear()

[jbuck2005]

'virtual void NetworkClient::flush()' is deprecated: Use clear() instead. [-Wdeprecated-declarations]

[Links2004]

looks like that the flush to clear change is only valide for the ESP32 3.x only way to do this then is to have some if defined for ESP32 and a check for ESP_ARDUINO_VERSION_MAJOR >= 3

clear is not part of default arduino API not sure why the ESP32 changed this... https://docs.arduino.cc/language-reference/funktionen/communication/stream/