correct SS pin to 21

Author: hathach

variants/adafruit_metro_esp32s3/pins_arduino.h

@@ -32,7 +32,7 @@ static const uint8_t RX = 41;
 static const uint8_t SDA = 47;
 static const uint8_t SCL = 48;
 
-static const uint8_t SS   = 42;
+static const uint8_t SS   = 21;
 static const uint8_t MOSI = 35;
 static const uint8_t SCK  = 36;
 static const uint8_t MISO = 37;