chore(examples): move the App Engine example to python311 (#1603)

Co-authored-by: Claude <svc-devxp-claude@slack-corp.com>
Co-authored-by: Eden Zimbelman <eden.zimbelman@salesforce.com>
Co-authored-by: Michael Brooks <mbrooks@slack-corp.com>

Author: 4 people

examples/google_app_engine/flask/app.yaml

@@ -1,4 +1,4 @@
-runtime: python38
+runtime: python311
 
 inbound_services:
   - warmup