Switch object storage proxy_download setting from true to false to enable redirects

What does this MR do and why?

Switches proxy_download from true to false after a regression.

Using an AWS S3 for object storage, the ~"Maven Repository" is not properly redirecting HEAD requests to object storage. When executing the redirect, it will end up in a 403 Forbidden response.

When GitLab needs to return a file that is present on object storage, the default (and ideal) configuration is: generate a pre signed url and redirect to that url. This redirect can be disabled with gitlab_rails['object_store']['proxy_download'] = true. By doing so, this issue disappears. (full investigation)

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