Edit Groups API annotation

This edits the file type in the documentation only as it is an edge case where API::Validations::Types::WorkhorseFile is not understood by grape-swagger and will be excluded from auto-generation. If also changes the success entity as Entities::UserPublic does not exist, which would cause OpenAPI auto-generation to fail as well.

The OpenAPI V2 documentation will be used for dogfooding API fuzzing as per #372422 (closed) and the OpenAPI autogeneration &8926.