httpcore2: prefer h2 for ALPN protocol if requesting HTTP/2

[benjarobin]

Summary

When building the list of ALPN protocols for negotiation in the SSL context:

• If HTTP/2 is requested, prioritize "h2" first. This is critical, as some servers (e.g., lighttpd) select the first protocol in the list.
• If HTTP/1.1 is not disabled, include "http/1.1". If HTTP/1.1 was disabled we do not want to advertise that, otherwise the server may select HTTP/1.1 as protocol in the SSL context, but the client (httpx2) is still going to send the request using HTTP/2, which is going to confuse the server.
• If both HTTP/2 and HTTP/1.1 are disabled, still include "http/1.1" as a fallback.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
  - I was not able to find the test infrastructure to test against an server with TLS and HTTP/2.
  - Maybe somebody could point me in the right direction.
• I've updated the documentation accordingly.
  - It does not seem necessary to update the documentation.

I will try to add test and to update the documentation if nobody is against this change :)

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 7 skipped benchmarks¹

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_{Comparing benjarobin:fix-alpn-protocol (368ff61) with main (5465b4e)}

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. ↩

[cubic-dev-ai]

No issues found across 6 files

_{Re-trigger cubic}