Update community links to pydantic/httpx2

[Kludex]

Summary

• Point issue template discussions URL at pydantic/httpx2.
• Drop the old encode gitter chat link from the issue template config.
• Update the PR template greeting from HTTPX to HTTPX2.
• Route Code of Conduct reports to hello@pydantic.dev instead of the personal email.

Test plan

• Confirm no remaining references to encode/httpx or tom@tomchristie.com in the touched files.

AI Disclaimer

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

[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing update-community-links (b7c977f) with main (8a903c6)}