chore(google-auth): drop python 3.7 EOL false positives and refactor metrics telemetry

[chalmerlowe]

This pull request eliminates EOL Python 3.7 false positives from the google-auth codebase to ensure a clean signal from version scanner compliance checks.

Why these changes are made:

• Metrics Telemetry: Hardcoded Python versions in telemetry headers (like gl-python/3.7) have been replaced with abstract version placeholders (<python-version>) in comments with formatting examples and in test assertions. This prevents scanner alerts while retaining the ability to verify HTTP client request formatting behavior.
• App Engine Runtime Tests: GAE standard runtime test cases have been refactored to dynamically construct GAE runtime values from the active Python interpreter at test execution time, avoiding the need for EOL checks and manual updates when Python versions retire.
• Clean Up Transport Properties: Replaced legacy private _auto_decompress internal attribute access in the aiohttp transport with the standard public auto_decompress property (supported in aiohttp >= 3.8), allowing the removal of old TODOs from the source code that referenced 3.7.
• Readme Cleanup: Removed the manual, out-of-date historical "Unsupported Python Versions" list from the package documentation, relying instead on standard authoritative packaging metadata (python_requires) to enforce runtime compatibility.

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Supports resolution of the internal bug: #512225398

[gemini-code-assist]

Code Review

This pull request cleans up Python 3.7 references across documentation, metrics comments, and tests, updating them to use dynamic versioning or placeholders. It also updates aiohttp session checks to use the public auto_decompress attribute instead of the private _auto_decompress. Feedback on these changes suggests using getattr for safer attribute access on potentially mocked aiohttp sessions, and correcting minor typos and duplications in the updated metrics comments.