Merge topic 'policy-version-var' into release-4.0

1a35351125 Add CMAKE_POLICY_VERSION_MINIMUM to help configure outdated projects
d723198539 Help: De-duplicate policy version documentation
e5d29e9e00 Help: Drop cmake_minimum_required pre-2.4 behavior

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Merge-request: !10343