chore: deprecate FILES_REST in favour of RDM_FILES

[carlinmack]

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Description

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This was deprecated in RDM records in August 2024
https://github.com/inveniosoftware/invenio-rdm-records/pull/1789/changes

We should've updated the docs at that point but it was missed

I think this could be improved a bit further still, but just pushing what I have at end of day :)

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• If this documentation change impacts the current release of InvenioRDM, I will backport it to the production branch following approval or indicate to a maintainer that it should be backported.

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