Set LEVEL_DETECTOR_SPL_8BIT_000_POINT to 52.0 dB.

[microbit-carlos]

This is the value currently used by MakeCode:
https://github.com/microsoft/pxt-microbit/blob/v7.1.3/libs/microphone/microphone.cpp#L8

And MicroPython:
https://github.com/microbit-foundation/micropython-microbit-v2/blob/v2.1.2/src/codal_app/codal.json#L16

The reason to change it is that the LevelDetectorSPL() minValue default parameter is already 52:

codal-core/inc/streams/LevelDetectorSPL.h

Lines 123 to 126 in 509086c

	LevelDetectorSPL(DataSource &source, float highThreshold, float lowThreshold, float gain,
	float minValue = 52,
	uint16_t id = DEVICE_ID_SYSTEM_LEVEL_DETECTOR_SPL,
	bool activateImmediately = true);

MicroBitAudio also sets it to the same value 52.0:
https://github.com/lancaster-university/codal-microbit-v2/blob/v0.2.68/source/MicroBitAudio.cpp#L82

And LevelDetectorSPL will clamp as that being the min.

codal-core/source/streams/LevelDetectorSPL.cpp

Lines 142 to 147 in 509086c

	if (conv < minValue)
	level = minValue;
	else if (isfinite(conv))
	level = conv;
	else
	level = minValue;

Fixes lancaster-university/codal-microbit-v2#449.