Return wxWinVersion_[78] correctly from wxMSW wxGetWinVersion().

This is especially important because the workaround of r76152 for IFileDialog
bug under Windows Vista also applied under later Windows versions as they were
not detected correctly.

Closes #16286.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76600 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775