Mark httpx2 and httpcore2 classifiers as Production/Stable

[Kludex]

Promote the Development Status trove classifier to 5 - Production/Stable for both packages:

• httpx2: was 4 - Beta
• httpcore2: was 3 - Alpha

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

[github-actions]

Docs preview:

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing classifier-production-stable (3458c1b) with main (26dd31e)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩