Misalignment between card tag validation and docs

[albertvillanova]

Describe the bug

As pointed out in other issue: #4827 (comment)
the validation of the dataset card tags is not aligned with its documentation: e.g.

• implementation: license: List[str]
• docs: license: Union[str, List[str]]

They should be aligned.

CC: @julien-c

[julien-c]

(Note that the doc is aligned with the hub validation rules, and the "ground truth" is the hub validation rules given that they apply to all datasets, not just the canonical ones)

[mariosasko]

Instead of our own implementation, we now use huggingface_hub's DatasetCardData, which has the correct type hint, so I think we can close this issue.