Fix mbed not using REFRESH_INTERVAL

[razvanphp]

This pull request refactors the Servo library for mbed-based boards to utilize std::chrono durations instead of manual conversions with floating-point numbers. The 20ms interval was hardcoded for mbed platform only.

Changes:

• Fix bug on mbed platform of not using REFRESH_INTERVAL at all
• Modified ServoImpl::start() and ServoImpl::call() to use std::chrono::microseconds.
• Eliminated floating-point divisions by 0.02f (hardcoded) and 1e6.
• Ensured backward compatibility by keeping existing functionality intact.

Benefits:

• Enhanced type safety and precision.
• Simplified codebase and aligned with other platforms.
• Facilitates dynamic adjustment of REFRESH_INTERVAL.

Testing:

• Verified functionality with default REFRESH_INTERVAL of 20000 microseconds.
• Tested with REFRESH_INTERVAL set to 10000 and 5000 microseconds.
• Confirmed correct servo operation and signal timing using an oscilloscope.

Notes:

This change aligns the Servo library with the updated mbed OS API that favours std::chrono durations since mbedOS 6.0.0, see

https://github.com/arduino/ArduinoCore-mbed/blob/702daa02e244fb8710b48599f38786a67c590699/cores/arduino/mbed/drivers/include/drivers/Ticker.h#L85-L122

[github-actions[bot]]

Memory usage change @ fab06090cb7e3cd20e0ed7bd7d9a22619e770766

Board	flash	%	RAM for global variables	%
arduino:avr:leonardo	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:avr:mega	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:avr:nano	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_nano:nano33ble	:green_heart: -448 - -448	-0.05 - -0.05	0 - 0	0.0 - 0.0
arduino:mbed_nano:nanorp2040connect	:green_heart: -1868 - -1868	-0.01 - -0.01	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A
arduino:mbed_portenta:envie_m7:target_core=cm4	N/A	N/A	N/A	N/A
arduino:megaavr:nona4809	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:sam:arduino_due_x_dbg	0 - 0	0.0 - 0.0	N/A	N/A
arduino:samd:mkrzero	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0

Click for full report table

Board|`examples/Knob`
flash|%|`examples/Knob`
RAM for global variables|%|`examples/Sweep`
flash|%|`examples/Sweep`
RAM for global variables|% -|-|-|-|-|-|-|-|- `arduino:avr:leonardo`|0|0.0|0|0.0|0|0.0|0|0.0 `arduino:avr:mega`|0|0.0|0|0.0|0|0.0|0|0.0 `arduino:avr:nano`|0|0.0|0|0.0|0|0.0|0|0.0 `arduino:mbed_nano:nano33ble`|-448|-0.05|0|0.0|-448|-0.05|0|0.0 `arduino:mbed_nano:nanorp2040connect`|-1868|-0.01|0|0.0|-1868|-0.01|0|0.0 `arduino:mbed_portenta:envie_m7`|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A `arduino:mbed_portenta:envie_m7:target_core=cm4`|N/A|N/A|N/A|N/A|N/A|N/A|N/A|N/A `arduino:megaavr:nona4809`|0|0.0|0|0.0|0|0.0|0|0.0 `arduino:sam:arduino_due_x_dbg`|0|0.0|N/A|N/A|0|0.0|N/A|N/A `arduino:samd:mkrzero`|0|0.0|0|0.0|0|0.0|0|0.0

Click for full report CSV

``` Board,examples/Knob
flash,%,examples/Knob
RAM for global variables,%,examples/Sweep
flash,%,examples/Sweep
RAM for global variables,% arduino:avr:leonardo,0,0.0,0,0.0,0,0.0,0,0.0 arduino:avr:mega,0,0.0,0,0.0,0,0.0,0,0.0 arduino:avr:nano,0,0.0,0,0.0,0,0.0,0,0.0 arduino:mbed_nano:nano33ble,-448,-0.05,0,0.0,-448,-0.05,0,0.0 arduino:mbed_nano:nanorp2040connect,-1868,-0.01,0,0.0,-1868,-0.01,0,0.0 arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A arduino:mbed_portenta:envie_m7:target_core=cm4,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A arduino:megaavr:nona4809,0,0.0,0,0.0,0,0.0,0,0.0 arduino:sam:arduino_due_x_dbg,0,0.0,N/A,N/A,0,0.0,N/A,N/A arduino:samd:mkrzero,0,0.0,0,0.0,0,0.0,0,0.0 ```

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