west.yaml: MCUboot synchronization from upstream

Update Zephyr fork of MCUboot to revision:
  6a8746d7acd75ff81fb6888d7776f412663f7897

Brings following Zephyr relevant fixes:
 - 6a8746d boot_serial: fix image number handle in image upload
           request
 - f2cb550 boot_serial: fix misuse of 'matched' param from
           zcbor_map_decode_bulk()
 - 82feb9a boot_serial: Fix showing images that are not valid
 - 61962b9 bootutil: fix FIH int conversion for security_cnt
 - e6e4801 zephyr/boot_serial_extension: Fix zcbor header path
 - a5db515 bootutil/crypto: SHA256 abort function return state
 - 0361ad3 bootutil/crypto: SHA256 init functions should return a
           status
 - f92a219 bootutil/crypto: Fix minor typos in comments for RSA
           modules
 - 4854700 bootutil: Add image_index to additional logging messages
 - 2f85b7e bootutil/crypto: Fix the common.h header
 - c321a70 bootutil/crypto: Add a crypto backend for SHA256 based
           on PSA Crypto APIs
 - 02bf072 bootutil/crypto: Refactor the RSA signature verification
           and encryption
 - ba5fb1c bootutil: Add image_index to common prints

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>