Add throttling configuration for Files API endpoints

What does this MR do and why?

Contributes to #338903 (closed)

It removes files_api_throttling feature flag and allows to throttle Files API endpoints.

Screenshots or screen recordings

How to set up and validate locally

1. Go to Admin Area -> Network: http://<gdk_url>/admin/application_settings/network
2. Enable a restrictive rate-limit for Files API and save changes. For example, 2 authorized requests per 15 seconds
3. Restart GDK to apply the rate-limit (gdk restart rails-web)
4. Make consecutive requests to Files API (for example, http://<gdk_url>/api/v4/projects/4/repository/files/README?ref=master)
5. Verify that API returns 429 page when rate-limit reached

MR acceptance checklist

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