chore: remove Python 3.9 from default nox session (#17017)

This PR removes Python 3.9 from the default nox session which is used in
continuous builds. Python 3.9 unit tests are still running in presubmits
via Github actions but cannot be tested in continuous builds since
Python 3.9 has been removed from the testing image as of
googleapis/testing-infra-docker#524.

Author: parthea

packages/google-cloud-spanner/noxfile.py

@@ -71,7 +71,6 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",