Settle on the supported versions of VTK for current MayaVi

Update : We can definitely drop support for the old pipeline. See @prabhuramachandran's comment here - https://github.com/enthought/mayavi/issues/1082#issuecomment-932784187

It is not clear at the moment what versions of VTK are supported by the current (unreleased) versions of MayaVi. It would be good to settle this matter - and once decided, update the CI jobs to test against all supported versions of VTK. See https://github.com/enthought/mayavi/pull/1076 for the new GitHub Actions CI jobs that are being setup to replace the Travis and AppVeyor CI jobs.