Release changing npm package upload from sync to async

What does this MR do and why?

In Add worker to upload npm packages async (!146493 - merged), we changed how we upload packages to the npm Registry from sync (during the request) to async (in a background sidekiq job).

The feature was gated behind a feature flag, and it was globally enabled for about two days now without issues.

So this MR is to clean up the feature flag and release the feature in milestone 17.0 (It's a breaking change and should be release in it).

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Related to #433009 (closed)