feat(dns): Drop support for Python 3.7, 3.8, and 3.9

[chalmerlowe]

This PR updates `google-cloud-dns` to establish Python 3.10 as the minimum supported version, dropping support for Python 3.7, 3.8, and 3.9.

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.
• Constraints: Transferred lower bounds to `constraints-3.10.txt` and dropped `constraints-3.9.txt`.

Verified successfully with 65 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-dns package. Key changes include updating documentation in CONTRIBUTING.rst and README.rst, removing Python 3.9 from the Nox test sessions, and updating the python_requires metadata in setup.py. Additionally, the testing constraints were updated from version 3.9 to 3.10, and obsolete deprecation warning filters for Python 3.7 were removed from pytest.ini. I have no feedback to provide as there were no review comments.