Fix accessibility of Whats New drawer headers

What does this MR do and why?

Fix accessibility of Whats New drawer headers

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Follow-up from "Use tailwind util classes for s... (#474113 - closed)
• ✂️ UX Paper Cuts 17.10 → Accessibility & AI Age... (#512691 - closed)

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch in the GDK
2. Visit the GDK
3. Open the "What's new" drawer by going to Help > What's new in the bottom of the left sidebar
4. Inspect the "What's new" header in dev tools. You will see that it uses an H2 semantic element.
5. Inspect the link to a feature in dev tools. You will see that it is a link inside an H3 semantic element.

Related to #474113 (closed)