Expand String examples with explicit size concat and constructor

[per1234]

Moved from https://github.com/arduino/Arduino/pull/9239 by @matthijskooijman

This expands examples to show the newly added APIs:

String::concat(const char *, unsigned int)
String::String(const char *, unsigned int)

Since not all cores versions will support this right away, a version check is added against ARDUINO_CORE_API.

These APIs are added to ArduinoCore-API in https://github.com/arduino/ArduinoCore-API/pull/97. Before merging these examples, the version check must be updated to the first API version that includes these new APIs.

[CLAassistant]

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