doc: cmake 3.20.0-rc1 presets version 2 required
The docs for cmake-presets indicate that
Currently, the only supported version is 1.
However with a minimal CMakePresets.json having a testPresets
array, an error results
ctest --preset default
CMake Error: Could not read presets from <sourcedir>: File version must be 2 or higher for build and test preset support.