Raise CMake requirements to 3.16

This is what the core modules currently require.

CHANGELOG.md

@@ -16,6 +16,7 @@
 - The old implementation of `LocalFEStiffness` (the one *not* in the `Dune::Elasticity` namespace)
   has been removed.
 
+- The module requires CMake 3.16 now.  This is what the Dune core modules currently require.
 
 # Release 2.9
 

CMakeLists.txt

-# We require version CMake version 3.1 to prevent issues
-# with dune_enable_all_packages and older CMake versions.
-cmake_minimum_required(VERSION 3.1)
+cmake_minimum_required(VERSION 3.16)
 project(dune-elasticity CXX)
 
 if(NOT (dune-common_DIR OR dune-common_ROOT OR