[cmake] Require CMake 3.16 or newer

CMakeLists.txt

 # SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file LICENSE.md in module root
 # SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
 
-cmake_minimum_required(VERSION 3.13)
+cmake_minimum_required(VERSION 3.16)
 project(dune-istl C CXX)
 
 # guess build tree of dune-common

python/CMakeLists.txt

@@ -7,6 +7,3 @@ dune_python_configure_bindings(
   PATH "."
   CMAKE_METADATA_FLAGS DUNE_OPTS_FILE
 )
-if(POLICY CMP0087)
-  cmake_policy(SET CMP0087 NEW)
-endif()