Update What's New Feature - article read status

What does this MR do and why?

Implement the unread indicator for the latest feature articles in what's new. Clicking the feature toggle hides the unread indicator.

Related to #550948 (closed)

References

Screenshots or screen recordings

Before	After
	Screen_Recording_2025-08-21_at_3.46.37_PM

How to set up and validate locally

1. Enable whats_new_featured_carousel feature flag (gdk rails c then Feature.enable(:whats_new_featured_carousel)
2. Login to gdk: https://docs.gitlab.com/development/ee_features/#simulate-a-saas-instance
3. On the bottom left, click What's new
4. Observe changes to whats new drawer

MR acceptance checklist

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