Deprecate legacy_cache_layout parameter in hf_hub_download

[Wauplin]

From a suggestion from @julien-c. It's not used since Aug 2022 so time to remove it for real I think? :)

This PR deprecates:

• filename_to_url
• url_to_filename
• cached_download
• legacy_cache_layout parameter

Deprecation is expected for release 0.26.

I took care of fixing HF libraries that had remaining occurrences. None of them were in highly-used part of the code anyway.

• Switch from cached_download to hf_hub_download in tests tokenizers#1547
• Switch from cached_download to hf_hub_download in remaining occurrences transformers#31284
• Move away from cached_download diffusers#8419

[HuggingFaceDocBuilderDev]

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[LysandreJik]

Ok!