Some vtk-packages are not available anymore in ubuntu 20.04

[chhtz]

From these

    default:
        - libvtk6-dev
        - tcl-vtk6
        - python-vtk6
        - libvtk6-java

only libvtk6-dev is still available. It seems to work if I upgrade this to

        - libvtk7-dev
        - tcl-vtk7
        - python3-vtk7
        - libvtk7-java

Is the proper way to fix this:

• Rename current default to 18.04, and add a new default with the entries above, or
• Create a new 20.04 (maybe also 22.04) entry with the new values?

I also don't know if the API changed somewhere and some dependencies would break -- but the version had been updated from vtk5 before, so hopefully this is fine.