Python 3.8 has EoL'ed. Update README to deprecate Python 3.8 support (#873)

Updated the 'Supported Python Versions' section in README.md to indicate that Python 3.7 and Python 3.8 support is deprecated, advising users to use Python 3.9 or higher.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: lahirumaramba

README.md

@@ -43,8 +43,8 @@ requests, code review feedback, and also pull requests.
 
 ## Supported Python Versions
 
-We currently support Python 3.7+. However, Python 3.7 support is deprecated,
-and developers are strongly advised to use Python 3.8 or higher. Firebase
+We currently support Python 3.7+. However, Python 3.7 and Python 3.8 support is deprecated,
+and developers are strongly advised to use Python 3.9 or higher. Firebase
 Admin Python SDK is also tested on PyPy and
 [Google App Engine](https://cloud.google.com/appengine/) environments.