fix(spanner): drop support for Python 3.9

[sakthivelmanii]

Python 3.9 is EOL and missing from the Kokoro test image, causing nightly failures.

• Remove "3.9" from noxfile.py ALL_PYTHON and sessions
• Remove "3.9" from setup.py classifiers and update python_requires to >=3.10

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9 in the google-cloud-spanner package. The changes involve removing Python 3.9 from the Nox configuration and updating setup.py to require Python 3.10 or higher. I have no feedback to provide as there were no review comments.

[parthea]

The files in this PR are automatically generated and patched using post processing here :

https://github.com/googleapis/google-cloud-python/blob/main/.librarian/generator-input/client-post-processing/spanner-integration.yaml

The changes to drop 3.9 in all packages which have files generated by gapic-generator will be rolled out next week.

Please can you file a bug with the specific presubmits that are failing today? Checks in the last release PR succeeded: #16964

[sakthivelmanii]

@partheahttps://fusion2.corp.google.com/ci;prev=s/kokoro/prod:cloud-devrel%2Fclient-libraries%2Fpython%2Fgoogleapis%2Fpython-spanner%2Fcontinuous%2Fcontinuous/activity/cad8210e-8fd3-4d22-9dfd-bcb85f4ab776/summary
our continuous kokoro job is failing due to this.

[parthea]

This will be addressed in #17017