Rename zero-shot pipeline multi_class argument
Renames the `multi_class` argument to `multi_label` in the `ZeroShotClassificationPipeline` and adds a deprecation warning to the former. Typically, "multi-label classification" is used to refer to this type of classification (where each class is evaluated independently).

The name is changed in the zero-shot distillation script as well. Resolves #6668.