fix(client): Make flush() call clear() and deprecate it (#10242)

This is a compromise for issues caused by https://github.com/espressif/arduino-esp32/pull/9453

libraries/Network/src/NetworkClient.cpp

@@ -369,7 +369,9 @@ int NetworkClient::read() {
   return data;
 }
 
-void NetworkClient::flush() {}
+void NetworkClient::flush() {
+  clear();
+}
 
 size_t NetworkClient::write(const uint8_t *buf, size_t size) {
   int res = 0;

libraries/Network/src/NetworkClient.h

@@ -56,6 +56,7 @@ public:
   size_t write(const uint8_t *buf, size_t size);
   size_t write_P(PGM_P buf, size_t size);
   size_t write(Stream &stream);
+  [[deprecated("Use clear() instead.")]]
   void flush();  // Print::flush tx
   int available();
   int read();

libraries/Network/src/NetworkUdp.cpp

@@ -288,7 +288,9 @@ size_t NetworkUDP::write(const uint8_t *buffer, size_t size) {
   return i;
 }
 
-void NetworkUDP::flush() {}
+void NetworkUDP::flush() {
+  clear();
+}
 
 int NetworkUDP::parsePacket() {
   if (rx_buffer) {

libraries/Network/src/NetworkUdp.h

@@ -64,6 +64,7 @@ public:
   int endPacket();
   size_t write(uint8_t);
   size_t write(const uint8_t *buffer, size_t size);
+  [[deprecated("Use clear() instead.")]]
   void flush();  // Print::flush tx
   int parsePacket();
   int available();