doc: update migration guide 4.2 for STM32 HAL Ethernet changes

Removed the Kconfig option `ETH_STM32_HAL_MII`.
PHY interface type should now be set using
the `phy-connection-type` property in the device tree.

Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <[email protected]>

Author: marwaiehm-st

doc/releases/migration-guide-4.2.rst

@@ -32,6 +32,9 @@ Boards
 Device Drivers and Devicetree
 *****************************
 
+* Removed Kconfig option ``ETH_STM32_HAL_MII`` (:github:`86074`).
+  PHY interface type is now selected via the ``phy-connection-type`` property in the device tree.
+
 Bluetooth
 *********