vtkm: add v1.6.0-rc1, make v1.5.1 preferred (#22855)
vtkm: add v1.6.0-rc1, make v1.5.1 preferred (#22855)

Signed-off-by: Vicente Adolfo Bolea Sanchez <50a1338ef7ca1a18e91552e384405b84a47e7740@kitware.com>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack