correct SS pin to 21, also add NEOPIXEL_PIN

Author: hathach

variants/adafruit_metro_esp32s3/pins_arduino.h

@@ -20,6 +20,7 @@
 #define LED_BUILTIN     13
 
 #define PIN_NEOPIXEL    45
+#define NEOPIXEL_PIN    45
 #define NEOPIXEL_NUM    1
 
 #define PIN_BUTTON1     0  // BOOT0 switch
@@ -32,7 +33,7 @@ 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;