doc: releases: update migration guide for stm32 adc properties

Update migration guide for STM32 ADC properties that now uses string
instead of int.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>

Author: gautierg-st

doc/releases/migration-guide-4.1.rst

@@ -94,6 +94,9 @@ STM32
   as it was introduced earlier.
   The Kconfig method for configuration is now removed.
 
+* ADC properties ``st,adc-sequencer`` and ``st,adc-clock-source`` now uses
+  string values instead of integer values.
+
 Modules
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