Use semantic headers for user stat cards in admin and unique links

What does this MR do and why?

• Use H2 elements for the stat card titles
• make sure the filter links have unique accesible text

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.

• #518121+
• ✂️ 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
2. Visit the admin users dashboard in the GDK
3. Use devtools to inspect the small header text for the three stats at the top of the page (Pending approval, Administrators, Without two-factor authentication). Each should be rendered with an H2 element.
4. Use devtools to inspect the Filter list link at the bottom of each of the stats. Each should have a title attribute that is different from the other two.