Use cmake policy instead of minimum

[sethrj]

This is a follow-up to #51 and is related to some nasty obscure side effects of the policy alteration by the previous implementation, seen in celeritas-project/celeritas#1573.

It's unusual to use cmake_minimum_required in a package config file, since it can alter policy scope elsewhere (depending, of course, on the policy that's been set). The cmake_policy command correctly scopes the changes to the Config file, rather than applying the change to the rest of the user's code when called from inside find_package or find_dependency.

The updated usage mirrors the CMake-generated Targets.cmake file: assertions to check for the rough CMake version, then a scoped policy change for the file itself.