west.yml: Update OpenThread revision
west.yml: Update OpenThread revision

Update OpenThread revision (up to commit ab9c0a4 upstream).

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

galak/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr