Fix Group avatar API endpoint

What does this MR do?

Instead of using avatar.file.file, which is a private API when using object storage, rely on avatar.filename which is the public API to fetch the avatar filename regardless of it being in the object storage or locally in the system.

Fixes https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/2696867

Related to #334872 (closed)

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