Align more metadata with other repo types (models,spaces)
_The documentation is not available anymore as the PR was closed or merged._ I just set a default value (None) for the deprecated licenses and languages fields, which should fix most of the CI failures.

Note that the CI should still be red because you edited many dataset cards and they're still missing some content - but this is unrelated to this PR so we can ignore these failures thanks so much @lhoestq !! There's also a follow-up PR to this one, in #4613 – I would suggest to merge all of them at the same time and hope not too many things are broken 🙀 🙀  Alright merging this one now, let's see how broken things get
see also associated PR on the `datasets-tagging` Space: https://huggingface.co/spaces/huggingface/datasets-tagging/discussions/2 (to merge after this one is merged)