docs(sqla): Update README and CONTRIBUTING for Python 3.9 drop

Author: chalmerlowe

packages/sqlalchemy-bigquery/CONTRIBUTING.rst

@@ -239,17 +239,6 @@ Supported versions can be found in our ``noxfile.py`` `config`_.
 .. _config: https://github.com/googleapis/google-cloud-python/blob/main/packages/sqlalchemy-bigquery/noxfile.py
 
 
-We also explicitly decided to support Python 3 beginning with version 3.9.
-Reasons for this include:
-
--  Encouraging use of newest versions of Python 3
--  Taking the lead of `prominent`_ open-source `projects`_
--  `Unicode literal support`_ which allows for a cleaner codebase that
-   works in both Python 2 and Python 3
-
-.. _prominent: https://docs.djangoproject.com/en/1.9/faq/install/#what-python-version-can-i-use-with-django
-.. _projects: http://flask.pocoo.org/docs/0.10/python3/
-.. _Unicode literal support: https://www.python.org/dev/peps/pep-0414/
 
 **********
 Versioning

packages/sqlalchemy-bigquery/README.rst

@@ -55,6 +55,7 @@ Python >= 3.10, <= 3.14
 
 Unsupported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+.. version-scanner: ignore-next-line
 Python <= 3.9