fix(runtimeconfig): Drop support for Python 3.9

[chalmerlowe]

This PR updates `google-cloud-runtimeconfig` to establish Python 3.10 as the minimum supported version, dropping support for Python 3.9 and below.

Changes

• Configuration: Updated `setup.py` and `noxfile.py` to require Python 3.10+ and remove references to Python 3.9.
• Documentation: Updated `README.rst` and `CONTRIBUTING.rst` to reflect supported Python versions.
• Cleanup: Removed dead deprecation warnings from `pytest.ini`.
• Constraints: Transferred lower bounds to `constraints-3.10.txt` and dropped `constraints-3.9.txt`.

Verified successfully with 49 passing unit tests under Python 3.10!

Fixes internal issue: http://b/482126936 🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9 across the google-cloud-runtimeconfig package, updating documentation, CI configurations, and package metadata to reflect a minimum requirement of Python 3.10. Feedback highlights inconsistencies where Python 3.13 and 3.14 are documented as supported but are absent from the setup.py classifiers and noxfile.py test versions.

[gemini-code-assist]

The documentation lists Python 3.13 and 3.14 as supported, but these versions are not included in the setup.py classifiers or the UNIT_TEST_PYTHON_VERSIONS in noxfile.py. Please update those files to ensure consistency. Additionally, ensure that all version-specific link targets for 3.13 and 3.14 are correctly defined in this file to avoid broken references.

References

1. When updating the list of supported Python versions in documentation, ensure that all version-specific link targets are present and correctly defined to avoid broken references.