Remove dead links from major-performance-breakthroughs-upgrade-guide.md (#1982)

## Summary

Removed two broken questlab.pro links that were causing markdown link
check failures in CI.

## Changes

- Removed Airbnb's RSC migration results section (dead link)
- Removed Meta's developer portal migration section (dead link)

Both links were returning Status: 0 in the markdown link checker,
indicating they are no longer accessible.

## Testing

The markdown link checker should now pass without these dead links.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Updated the React Server Components performance upgrade guide by
removing certain migration case study references to streamline the
documentation.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude <[email protected]>

Author: justin808

docs/pro/major-performance-breakthroughs-upgrade-guide.md

@@ -27,15 +27,6 @@ Server Components execute on the server and stream HTML to the client—no serve
 - [geekyants.com Case Study](https://geekyants.com/en-gb/blog/boosting-performance-with-nextjs-and-react-server-components-a-geekyantscom-case-study):
   - **52% smaller** JavaScript and TypeScript codebase
   - Lighthouse scores improved **from ~50 to ~90**
-- [Airbnb's RSC migration results](<https://questlab.pro/blog-posts/web-development/wd-pl-2024-articleId912i1h212818#:~:text=%22Our%20migration%20to%20React%20Server%20Components%20resulted%20in%20a%2015%25%20improvement%20in%20core%20web%20vitals%20and%20a%2023%25%20reduction%20in%20Time%20to%20First%20Byte%20(TTFB)%20across%20all%20markets.%22%20%2D%20Airbnb%20Engineering%20Team>):
-- **15% improvement** in core web vitals
-- **23% reduction** in Time to First Byte
-- [Meta's developer portal migration](https://questlab.pro/blog-posts/web-development/wd-pl-2024-articleId912i1h212818#:~:text=Meta%27s%20RSC%20Implementation%20Results):
-  - **30% reduction** in JavaScript bundle size
-  - **60% improvement** in Time to Interactive
-  - **45% faster** First Contentful Paint
-  - **50% reduction** in server response time
-  - **25% decrease** in overall maintenance complexity
 
 Please note that only the first of these directly compares performance of equivalent applications with and without React Server Components.
 Other migrations may include React or other dependency upgrades and so on.