Fix SPISettings name collision in ArduinoISP sketch when compiled in bitbanged SPI mode for board using ArduinoCore-API

When using bit banged SPI, which the sketch did when compiled for any architecture other than AVR, a `SPISettings` class was declared by the sketch. At the time the sketch was written, it was reasonable to expect this would not cause a name collision, since SPI.h is not `#include`d when doing bit banged SPI. However, since then a `SPISettings` class has been declared in [ArduinoCore-API's HardwareSPI.h](https://github.com/arduino/ArduinoCore-API/blob/932c7c7d4d4d334b10484284cc846672ad59607c/api/HardwareSPI.h#L37), causing the ArduinoISP sketch to not compile for any board whose core uses ArduinoCoreAPI (currently Arduino Mega AVR Boards, "Arduino nRF528x Boards (Mbed OS]", and "Arduino Mbed OS Boards (nRF52840 / STM32H747)"):

```
  /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:191:27: error: reference to 'SPISettings' is ambiguous
       void beginTransaction(SPISettings settings) {
                             ^~~~~~~~~~~
  /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:167:7: note: candidates are: class SPISettings
   class SPISettings {
         ^~~~~~~~~~~
  In file included from /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/api/ArduinoAPI.h:31:0,
                   from /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/Arduino.h:23,
                   from /github/workspace/examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino:39:
  /github/home/.arduino15/packages/arduino/hardware/megaavr/1.8.6/cores/arduino/api/HardwareSPI.h:37:7: note:                 class arduino::SPISettings
   class SPISettings {
         ^~~~~~~~~~~
```

The fix is to use the `ARDUINO_API_VERSION` macro defined by ArduinoCore-API to detect when it is in use and make the bitbanged SPI code use the `SPISettings` class from ArduinoCore-API in this case.

Author: per1234

Diff for: examples/11.ArduinoISP/ArduinoISP/ArduinoISP.ino

@@ -164,19 +164,23 @@ void pulse(int pin, int times);
 
 #define SPI_MODE0 0x00
 
+#if !defined(ARDUINO_API_VERSION) // A SPISettings class is declared by ArduinoCore-API
 class SPISettings {
   public:
     // clock is in Hz
-    SPISettings(uint32_t clock, uint8_t bitOrder, uint8_t dataMode) : clock(clock) {
+    SPISettings(uint32_t clock, uint8_t bitOrder, uint8_t dataMode) : clockFreq(clock) {
       (void) bitOrder;
       (void) dataMode;
     };
 
-  private:
-    uint32_t clock;
+    uint32_t getClockFreq() const {
+      return clockFreq;
+    }
 
-    friend class BitBangedSPI;
+  private:
+    uint32_t clockFreq;
 };
+#endif  // !defined(ARDUINO_API_VERSION)
 
 class BitBangedSPI {
   public:
@@ -189,7 +193,7 @@ class BitBangedSPI {
     }
 
     void beginTransaction(SPISettings settings) {
-      pulseWidth = (500000 + settings.clock - 1) / settings.clock;
+      pulseWidth = (500000 + settings.getClockFreq() - 1) / settings.getClockFreq();
       if (pulseWidth == 0) {
         pulseWidth = 1;
       }