chore: make Python 3.9 minimum required build

Author: jackwotherspoon

.github/sync-repo-settings.yaml

@@ -44,7 +44,7 @@ branchProtectionRules:
   requiredStatusCheckContexts:
     - "Kokoro CI - Lint"
     - "Kokoro CI - Python 2.7 (App Engine Standard Only)"
-    - "Kokoro CI - Python 3.8"
+    - "Kokoro CI - Python 3.9"
     - "Kokoro CI - Python 3.12"
     - "cla/google"
     - "snippet-bot check"