chore: remove 3.9 from kokoro tests (#1163)

3.9 has been removed from the kokoro base image, so we should remove the
tests from our configs

3.9 is still tested in GitHub Actions

Author: daniel-sanche

packages/google-cloud-firestore/noxfile.py

@@ -85,7 +85,6 @@
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
 
 nox.options.sessions = [
-    "unit-3.9",
     "unit-3.10",
     "unit-3.11",
     "unit-3.12",