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 <marwa.ibenelhadjmessaoud-ext@st.com>