Change LSE oscillator drive capability to Medium low

[mvladic]

This could have a positive impact on the RTC precision.

More information:

RTC clock is using Low-Speed External (LSE) oscillator. There is a setting to specify drive capability:

From other source about this:

Up to now we used LOW setting. Only caveat is that changing to Medium Low could have impact on the battery lifetime - I didn't find any concreate info about this.

Here is an Application Note for the oscillator design:

https://www.st.com/resource/en/application_note/an2867-guidelines-for-oscillator-design-on-stm8afals-and-stm32-mcusmpus-stmicroelectronics.pdf

Currently, we don't have this on our PCB:

• The oscillator PCB must be underlined with a dedicated underneath ground plane,
  distinct from the application PCB ground plane.