What's New for 17.4 File Creation

What does this MR do and why?

• Adds the "what's new" content for the 17.4 release
• To be finalized on the Wednesday of release week (Sept 18th), and typically get merged by a maintainer 2 to 4 hours AFTER the release post is live on the release date (Sept 19th).
• Overview: https://handbook.gitlab.com/handbook/marketing/blog/release-posts/#creating-an-mr-for-whats-new-entries

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.