west.yml: Update mcumgr revision

Commits affecting Zephyr that are included with the new revision:
    4fa8691 zephyr: Allow to select slot for DFU as image
    d8802de samples/smp_svr/zephyr: Update stack size
    e71063a zephyr: Improve and rename img_mgmt_find_best_area_id
    e215d26 zephyr: Check area id in erase slot processing
    71c76f5 zephyr: Upload should use g_img_mgmt_state
    afa95ba cmd/img_mgmt: Call dfu stop cb on erase
    6a10fa6 img_mgmt: Add interpretation of "image" parameter
    757965c samples: smp_svr: zephyr: Update MTU Kconfig values

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>