Remove Python 3.10 compatibility shims

[shahar1]

My favorite PR in the series so far - this PR removes all 3.10 compatibility shims.

related: #74148

AI Summary

CI no longer runs Python 3.10 (layer 1) and the prek hooks run on 3.11 (layer 2), so these shims have no version left to serve:

• tomli fallbacks replaced by plain tomllib imports in breeze and the docs build. The same change in the prek scripts, dev/registry and docker-tests is deferred to layer 7: tomllib only sorts as a stdlib import once their pyproject.toml says 3.11, so doing it here would trip isort (I001).
• typing_extensions imports of Self, Unpack, assert_never in typing_compat, shared/dagnode and shared/observability replaced by typing.
• Task SDK BaseOperator.event_loop() uses asyncio.Runner directly; the 3.10 re-implementation and its _cancel_all_tasks helper are removed, and the test fixture that exercised the fallback goes with them.
• sys.version_info < (3, 11) test skips removed; comments that explained 3.10-specific isinstance(list[X], type) behaviour rewritten.

Stack

Layer 3 of 9 of the Python 3.10 drop (supersedes #74144, which carried the whole change in one PR). Each layer is meant to be green on its own: the script metadata bump comes first because prek picks the hook interpreter from it, and the ruff fixes land before the requires-python bump because ruff infers its target per distribution from the nearest pyproject.toml.

1. Drop Python 3.10 from CI, Breeze and the container images
2. Bump inline script metadata to Python 3.11
3. Remove Python 3.10 compatibility shims
4. Apply ruff Python 3.11 fixes to core, Task SDK, shared and airflowctl
5. Apply ruff Python 3.11 fixes to providers
6. Apply ruff Python 3.11 fixes to dev tooling, scripts and test suites
7. Require Python 3.11 in all distributions
8. Update provider READMEs for the Python 3.11 baseline
9. Update docs for the Python 3.11 baseline

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Fable 5.1)

Generated-by: Claude Code (Fable 5.1) following the guidelines