For policy-specific tests, use the version before the policy was
introduced.  Otherwise, use 3.5 where possible.

Also, remove `cmake_minimum_required()` and `project()` calls from
individual cases where they are handled by `CMakeLists.txt`.

The OLD behaviors of all policies are deprecated, but only by
documentation.  Add an explicit deprecation diagnostic for policies
introduced in CMake 3.18 and below to encourage projects to port
away from setting policies to OLD.

This is an old module from when VTK and other Kitware projects drove the
addition of commands and modules into CMake itself. Modern VTK doesn't
need this module and it can be ignored.

This module is kept around so that the ancient VTK versions which use it
are not broken by it. VTK itself stopped using the module in 2012 and
the last usage by an example in VTK was removed in 2020.

Fixes: #20591