#1274 Drop support for Python 3.10

Python 3.10 reaches end of life this month. This PR sets requires-python = ">=3.11" for both httpx2 and httpcore2.

• Remove 3.10 from the CI matrix and the trove classifiers.
• Set ruff target-version = "py311", which rewrites one asyncio.TimeoutError to TimeoutError.
• Drop the exceptiongroup fallback import and the 3.10-only netrc test with its fixture.
• Update the requirement in docs/migration.md.
• Re-lock, which removes exceptiongroup and async-timeout.

AI Disclaimer

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