fix(ndb): Drop support for Python 3.9

[chalmerlowe]

This PR updates `google-cloud-ndb` to drop support for Python 3.9 and establish Python 3.10 as the minimum supported version.

Changes

• Configuration: Updated `setup.py` and `noxfile.py` to require Python 3.10+ and remove references to Python 3.9.
• Documentation: Updated `README.md` and `CONTRIBUTING.rst` to reflect supported Python versions.
• Constraints: Transferred lower bounds to `constraints-3.10.txt` and dropped `constraints-3.9.txt`.
• Updated lower bound for pytz due to incompatibility with 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-ndb package, updating documentation, CI configurations (Nox), and package metadata to reflect a minimum requirement of Python 3.10. Feedback includes adding missing documentation links for Python 3.13 and 3.14 in CONTRIBUTING.rst and including the corresponding Trove classifiers in setup.py to match the supported version range.