Migrate response hashes in API::Files to Grape::Entity
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The following discussion from !103408 should be addressed:
- [ ] @igor.drozdov started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103408#note_1164669759): (+1 comment)
> @robotmay_gitlab could you please have a look at this merge request?
>
> Are these changes sufficient? Or do we need _at least_ `success Entity` entries?
>
> Currently, the endpoints return plain hashes, so if we can avoid converting them to entities, then I think we can skip it :thumbsup:
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