[cmake] Require CMake 3.16 or newer

This is related to core/dune-common!1302 (merged) which is related to or decision from the Dune developer meeting 2022, see https://dune-project.org/community/meetings/2022-02-devmeeting/#minimum-versions-of-gcc-clang-cmake-python

This MR is meant to keep the CMake version requirements in sync across multiple modules. Once the MR in dune-common is merged, this one should be merged, too.

cmake/modules/DuneAlugridMacros.cmake

-# do not ignore <PACKAGE>_ROOT variables
-cmake_policy(SET CMP0074 NEW)
-
 # from dune-grid/cmake/modules
 include(GridType)
 #define available alugrid types