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

Author: chalmerlowe

packages/google-cloud-core/CONTRIBUTING.rst

@@ -215,17 +215,6 @@ Supported versions can be found in our ``noxfile.py`` `config`_.
 .. _config: https://github.com/googleapis/google-cloud-python/blob/main/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/google-cloud-core/README.rst

@@ -36,5 +36,5 @@ Python >= 3.10
 
 Unsupported Python Versions
 ---------------------------
-
+.. version-scanner: ignore-next-line
 Python <= 3.9