cmake: boilerplate update comment to CMake 3.20
cmake: boilerplate update comment to CMake 3.20

boilerplate.cmake contained a comment referring to CMake 3.13.1 but
minimal required version is 3.20, so update the comment to 3.20.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

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