Revert "There is no upper bound to the cmake version, or if there was, it would be more than 3.20."

This reverts commit 0dbc9bc.

Actually, the <policy_max> indicate that a higher cmake
version should adopt a <policy_max> compatibility mode (unless
explicitly asked otherwise) and warn aboud post <policy_max>
behavior.

Author: aminiussi

CMakeLists.txt

@@ -3,7 +3,7 @@
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.5)
+cmake_minimum_required(VERSION 3.5...3.20)
 
 project(boost_mpi VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES C CXX)