Increase required CMake version to 3.16
Increase CMake version to 3.16. This was a decision from Dune developer meeting 2022, see https://dune-project.org/community/meetings/2022-02-devmeeting/#minimum-versions-of-gcc-clang-cmake-python
Increase CMake version to 3.16. This was a decision from Dune developer meeting 2022, see https://dune-project.org/community/meetings/2022-02-devmeeting/#minimum-versions-of-gcc-clang-cmake-python