fix(adc_test): correct ADC reference configuration

- Revert ADC_GAIN to ADC_GAIN_1 due to driver limitation.
- Revert ADC_REF to ADC_REF_INTERNAL due to driver limitation.
- Set zephyr,vref-mv to 3300 to match observed hardware behavior.

software/apps/adc_test/boards/weact_stm32g431_core.overlay

@@ -28,4 +28,4 @@
     adc1_in1_pa0: adc1_in1_pa0 {
         pinmux = <STM32_PINMUX('A', 0, ANALOG)>;
     };
-};
+};