Draft: tools: raise the minimum CMake version to 3.20

This should provide proper Android support. This version was released 2 years ago.

Extracted from !3278 (merged). Let's see if the new Android issues are due to using a newer CMake or using the NDK toolchain file.

The bad news is that all Docker images currently use an older CMake (Debian bullseye is using 3.18). So if this works, we should update the Docker images first to build a newer CMake version (using bullseye-backports). Otherwise we'll end up building CMake for every CI run, which will makes considerably slower. While doing that, we should do the same with Meson which is always rebuilt as well...