doc/releases/3.1: Add notes on MCUMGR to zcbor transition
doc/releases/3.1: Add notes on MCUMGR to zcbor transition

The commit adds release notes on MCUMGR transition from TinyCBOR
to zcbor, and deprecation of several Kconfig options that are
no longer needed.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

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