Migrate response hashes in API::Files to Grape::Entity

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The following discussion from !103408 (merged) should be addressed:

• @igor.drozdov started a discussion: (+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 👍