build: drop support for Python 3.9

[qianzhu18]

Changes

Python 3.9 is EOL, and the 0.43.0 RC1 release vote surfaced that the [cli]/[learn]/[start] extras were already broken on it (PEP 604 unions in burr/cli/__main__.py), forcing the release-validation smoke matrix to skip 3.9. This makes the 3.10 floor official everywhere:

• pyproject.toml: requires-python = ">=3.10" (no version-specific classifiers existed, so nothing else to drop there)
• CI: remove 3.9 from the four Python version matrices in .github/workflows/python-package.yml, and remove the now-obsolete 3.9 skip comment in .github/workflows/release-validation.yml
• docs: README note, docs/getting_started/install.rst, docs/contributing/setup.rst, and the website downloads page now state the 3.10+ requirement (the old "core remains compatible with 3.9" notes added by Bump to 0.43.0 #902 are superseded)
• scripts/README.md prerequisite and the shutil-filter compatibility comment in scripts/apache_release.py (3.9-3.11 → 3.10-3.11)

No runtime behavior changes — metadata, CI, and documentation only.

Fixes #900

How I tested this

• uv sync and uv build succeed with the new requires-python — sdist and wheel metadata build cleanly
• Both touched workflow files parse as valid YAML
• pytest tests/core/test_action.py tests/core/test_application.py tests/test_release_config.py: 235 passed / 61 failed — identical failure set on unmodified main under the same local interpreter (Python 3.14, above the supported ceiling), so the failures are pre-existing environment effects, not regressions
• grep -rn "3\.9" across toml/yml/rst/md/py confirms no remaining reference to 3.9 as a supported target (the remaining matches are either the historical 0.43.0 release note or langfuse>=3.9 package-version comparisons, both intentionally untouched)

Notes

• I deliberately did not touch the py<3.9 AST comments in burr/core/action.py — they predate this change (they are already below the old 3.9 floor) and are harmless permissive parsing
• SDIST and WHEEL naming and license cleanup #901 (sdist/wheel naming and license cleanup) intentionally left out — it overlaps scripts/apache_release.py/pyproject.toml with the open fix(release): address 0.43.0 RC feedback #882; happy to pick it up after that lands

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.