ci: Bump to use CMake 3.20.5

With CMake minimum required as 3.20.0 we update CI to use docker image
v0.18.2, which contains CMake 3.20.5.

For doc builds we fetch the same CMake v3.20.5 but using pip as the doc
build doesn't use the docker image.

The main reason for increasing CMake version is better toolchain
support.
The decision to bump the CMake version was taken by the Toolchain WG.

Better toolchain support is added in the following CMake versions:
- armclang, CMake 3.15
- Intel oneAPI, CMake 3.20
- IAR, CMake 3.15 and 3.20

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