Instance Configuration: Add rate limits for Users API

What does this MR do and why?

Instance Configuration: Add rate limits for Users API

This MR adds rate limits for the Users API to the instance configuration.

🛠️ with ❤️ at Siemens

References

• Update Instance Configuration to include newly ... (#575891)
• https://docs.gitlab.com/administration/settings/rate_limit_on_users_api/

Screenshots or screen recordings

How to set up and validate locally

1. To see the limit, go to Help > Check the current instance configuration or go to https://gdk.test:3443/help/instance_configuration#rate-limits
2. You might have to clear the cache with Rails.cache.clear in the rails console.
3. If you want to set or change the limit, go to Admin > Settings > Network > Users API rate limit..

MR acceptance checklist

MR Checklist ( @nwittstruck)

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the javascript style guides
• Conforms to the database guides

Related to #575891