Fix broken links to the React 19 Upgrade Guide

[see-stack]

Three links still used the old /blog/2024/04/25/react-19 path, which 404s since the page was renamed to react-19-upgrade-guide. Updated them to the current path.

[github-actions]

Size changes

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[deepusnath]

These links aren't currently broken — vercel.json (L248) permanently redirects /blog/2024/04/25/react-19 → /blog/2024/12/05/react-19, and the fragment is preserved, so they land on the right headings today.

The new target is the problem: react-19-upgrade-guide.md doesn't define {/error-handling/} or {/ref-as-a-prop/} — those headings live in blog/2024/12/05/react-19.md (L727 and L396). So after this change the pages load but drop the reader at the top of the wrong post, which a link checker won't flag.

If the goal is to skip the redirect hop, the repo already has a convention for that — useImperativeHandle.md:49 links straight to /blog/2024/12/05/react-19#ref-as-a-prop. Retargeting to that path instead would work.

[see-stack]

Thanks for catching this!