Adjust UART Clock to LSE for Low Power Mode

**Describe the Bug**
When the STM32 microcontroller is in low-power mode, the UART communication fails due to the current clock source. To work around this, low-power mode was disabled, which is not ideal. The root cause is that the UART clock source is incompatible with low-power mode.

**Expected Behavior**
The UART should work seamlessly in low-power mode by adjusting the clock source to LSE.

**Environment**
Operating System: Windows 11
PlatformIO Version: 6.1.15
Hardware: Soil Power Sensor version: 2.2.1

**Additional Context**
This issue impacts the ability to maintain power efficiency in devices where low-power operation is critical. The solution involves configuring the UART clock source to LSE and testing communication with low-power mode enabled.