cmake: recommend CMake >=3.20.5 as minimum CMake version

CMake versions <3.20.5 contains the following bug:
https://gitlab.kitware.com/cmake/cmake/-/issues/22310

This bug was fixed in CMake 3.20.5:
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/6232



Generally Zephyr can build with CMake >=3.20.0, however the
`cmake/package_helper.cmake` is impacted by the above issue.

Therefore, specifically request CMake >=3.20.5 for package helper
and update documentation to recommend CMake version 3.20.5 in order
to minimize risk of users being impacted by this bug.

Users invoking package helper with CMake 3.20.0-3.20.4 will see:
> CMake Error at cmake/package_helper.cmake:45 (cmake_minimum_required):
>  CMake 3.20.5 or higher is required.  You are running version 3.20.x

Rest of Zephyr still builds with CMake versions >=3.20.0.

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