FindVTK: Drop support for finding VTK 4.0

Convert to a simple thin-wrapper around the find_package(VTK NO_MODULE).
This makes the code much simpler and improves error messages.
Merge topic 'FindVTK-drop-4.0-support' into next

213f7f5f FindVTK: Drop support for finding VTK 4.0
Help: Add notes for FindVTK module updates
Merge topic 'FindVTK-drop-4.0-support' into next

3156f6af Help: Add notes for FindVTK module updates