Treat API requests from the frontend as web traffic in the rate limiter

What does this MR do and why?

This was previously behind a feature flag and disabled by default, and is now being enabled globally after verifying that this change doesn't break anything on gitlab.com.

How to set up and validate locally

Not really necessary anymore, but see !79341 (merged) for instructions, and #350623 (closed) for the verification results.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #350623 (closed)