...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.