chore(appengine): Update flex hello-world app outdated dependencies (#13807)

Author: amcolin

appengine/flexible/hello_world/app.yaml

@@ -18,6 +18,7 @@ entrypoint: gunicorn -b :$PORT main:app
 
 runtime_config:
   operating_system: ubuntu24
+  runtime_version: 3.12
 
 # This sample incurs costs to run on the App Engine flexible environment.
 # The settings below are to reduce costs during testing and are not appropriate

appengine/flexible/hello_world/requirements.txt

@@ -1,5 +1,2 @@
-Flask==3.0.3; python_version > '3.6'
-Flask==2.3.3; python_version < '3.7'
-Werkzeug==3.0.3; python_version > '3.6'
-Werkzeug==2.3.8; python_version < '3.7'
-gunicorn==23.0.0
+Flask==3.0.3
+gunicorn==22.0.0