Build cleanly under c++14

[richievos]

Other parts of TinyConsole require >= c++ 14, but this forces c++17.

That's not necessarily a problem, but it triggered some build warnings for me, so in case that's unintentional I'm sending this diff.

Exact warning:

Compiling .pio/build/esp32dev/lib4a7/TinyMqtt/TinyMqtt.cpp.o
.pio/libdeps/esp32dev/TinyMqtt/src/TinyMqtt.cpp: In member function 'MqttError MqttBroker::subscribe(MqttClient*, const Topic&, uint8_t)':
.pio/libdeps/esp32dev/TinyMqtt/src/TinyMqtt.cpp:221:13: warning: structured bindings only available with -std=c++17 or -std=gnu++17
   for(auto& [retained_topic, retain]: retained)

I personally just switched to c++17 as my fix/workaround.

[hsaturn]

Thanks for this.