Bump vtk 9.6.1

[jschueller]

as needed by paraview

/cc @traversaro @Tobias-Fischer

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The recipe is not parsable by parser conda-recipe-manager. The recipe can only be automatically migrated to the new v1 format if it is parseable by conda-recipe-manager.

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[hmaarrfk]

Please confirm why you intend to skip the migration

[jschueller]

oh, right, there's probaly a good reason why its pinned down to the patch version