Dune "policy" version numbers are different from meaning in CMake policies

The CMake policies associate a policy with a CMake version when they were introduced. This allows to have a command cmake_policy(VERSION ...) to set all policies introduced in a specified CMake version range to the value NEW.

In Dune, we also have a version associated to dune policies. This version is used to indicate when to switch to the NEW behavior automatically. In CMake they just state that the OLD behavior is deprecated by definition and may be removed in a future version of CMake.

Should we switch to a behavior like in CMake or stick with our interpretation? Currently there are no dune policies defined yet, so we could still switch in the behavior.