Tone limit 20KHz (#7179)

Set Tone maximum frequency to 20KHz (was 5KHz)

Author: Tech-TX

cores/esp8266/Tone.cpp

@@ -35,8 +35,8 @@ static void _startTone(uint8_t _pin, uint32_t high, uint32_t low, unsigned long
 
   pinMode(_pin, OUTPUT);
 
-  high = std::max(high, (uint32_t)100);
-  low = std::max(low, (uint32_t)100);
+  high = std::max(high, (uint32_t)25);  // new 20KHz maximum tone frequency,
+  low = std::max(low, (uint32_t)25);   // (25us high + 25us low period = 20KHz)
 
   if (startWaveform(_pin, high, low, (uint32_t) duration * 1000)) {
     _toneMap |= 1 << _pin;