feat(spanner): drop Python 3.7-3.9 support and regenerate

[chalmerlowe]

Updates post processing to account for dropping support for Python 3.7, 3.8, 3.9 and the impacts that has on using 3.10 for lower bounds testing.

Changes

• updates the lower bound versions for several libraries to avoid conflicts and install issues in both setup.py and constraints-3.10.txt
• updates post-processing scripts to ensure the above updates persist

[gemini-code-assist]

Code Review

This pull request drops support for Python 3.9 across the Spanner client library, updating CI configurations, dependency constraints, and package initialization logic. However, several issues were identified in the review: the documentation in README.rst files incorrectly suggests that Python 3.9 is still supported while 3.8 is the limit, and the setup.py file contains inconsistent dependency downgrades for google-api-core and proto-plus. Additionally, missing f-string prefixes in __init__.py files for both database and instance admin packages will prevent proper variable interpolation in warning messages. Feedback also suggests ensuring that setup.py metadata (classifiers and python_requires) and the package versioning reflect this breaking change.