opta/tutorials/mac-address: add note for older bootloader versions

[Channel59]

Add a note that older bootloader versions don't have the version number stored at this address, and the code of this example will not work and return v255.

What This PR Changes

• This PR adds a note in the Arduino docs, that the value for the bootloader version as reported by the docs is not always valid (See this issue: https://github.com/arduino/ArduinoCore-mbed/issues/887).

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