[docs] doc: fix stale Python version and CI references across doc/

[github-actions]

Summary

Reviewed all Markdown files under doc/ for inconsistencies with the actual source code and repository structure. Found and fixed three files where documentation still referred to Python 3.8 or 3.6 support, contradicting the real minimum requirement of Python 3.9 enforced by:

• sdk/core/azure-core/pyproject.toml → requires-python = ">=3.9"
• sdk/storage/azure-storage-blob → requires-python = ">=3.9"
• sdk/identity/azure-identity/pyproject.toml → requires-python = ">=3.9"
• eng/tools/azure-sdk-tools/pyproject.toml → python_requires=">=3.9"
• doc/tool_usage_guide.md (already correct) → "The monorepo requires a minimum of python 3.9"

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Changes

doc/dev/tests.md

• Changed "compatible with Python 3.8 and up" → 3.9 and up.
• Removed the stale Python 3.8 Windows Store link; updated the remaining list to start at 3.9.
• Updated the example venv path from Python38/python.exe py38_venv → Python39/python.exe py39_venv.

doc/dev/packaging.md

• Changed "Wheels must work with Python 3.8+" → 3.9+.
• Updated the inline python_requires snippet from ">=3.8" → ">=3.9".
• Removed the Programming Language :: Python :: 3.8 classifier from the full setup.py example and updated python_requires there as well.
• In the "migrate from Python 2" checklist, updated the python_requires example to ">=3.9" and expanded the classifier removal note to include 3.7 and 3.8 (not just 3.5/3.6).

doc/dev/engineering_assumptions.md

• Removed the stale snippet showing [bdist_wheel] universal=1 in setup.cfg — the repo is Python 3-only and the universal flag must not be set.
• Replaced the claim that "wheels are currently generated by a CI step running Python 3.6" with accurate language: wheels are generated by CI steps running Python 3.9+.
• Clarified that not having universal is the correct state for all packages.

Generated by Documentation Consistency Check · ◷

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