Stop generating vtk wheel for Python 3.8 and 3.9

Hello everyone,

As the Python version 3.8 is end of life since one year, we plan to stop generating VTK wheel for this version.

Additionally, once Python 3.9 will be end of life at the end of this month, we will also remove it soon.

Note that, VTK will still be compatible with these versions, only wheels automatically generated by Kitware are impacted.