components: lwip - modify HEAP_HIGHWAT from 6K to 20K

Since esp32 has much more memory than esp31B, modify the HEAP_HIGHWAT from
6K to 20K.
Notes: HEAP_HIGHWAT is used for tcp/udp tx flow control, if socket task detects
that the free heap memory is less than HEAP_HIGHWAT, it stops to trasmitting until
the free heap memory is greater than HEAP_HIGHWAT

components/lwip/include/lwip/port/lwipopts.h

@@ -514,7 +514,7 @@ extern unsigned char misc_prof_get_tcp_snd_buf(void);
 #define CHECKSUM_CHECK_UDP              0
 #define CHECKSUM_CHECK_IP               0
 
-#define HEAP_HIGHWAT                    6*1024
+#define HEAP_HIGHWAT                    20*1024
 
 #define LWIP_NETCONN_FULLDUPLEX         1
 #define LWIP_NETCONN_SEM_PER_THREAD     1