AttributeError: 'DefaultApiStub' object has no attribute 'CancelApiCalls'

## Expected Behavior

Upgrading from Python2 to Python3, I expected the following piece of code to continue working with this library.

```
try:
from google.appengine.api import runtime, apiproxy_stub_map

def hook():
logging.info("Instance is shutting down; cleaning up.")

apiproxy_stub_map.apiproxy.CancelApiCalls()

runtime.set_shutdown_hook(hook)
except ImportError:
pass
```

Is this deprecated? If yes, then should it be replaced with something else or is this functionality not needed any more? I can see the method somewhere in the library source code, but AppEngine seems not recognizing it?

## Actual Behavior

```
Traceback (most recent call last):
File "/layers/google.python.pip/pip/lib/python3.12/site-packages/google/appengine/runtime/middlewares.py", line 140, in ErrorLoggingMiddleware
return app(wsgi_env, start_response)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.12/site-packages/google/appengine/runtime/middlewares.py", line 82, in
lambda app: lambda wsgi_env, start_resp: f(app, wsgi_env, start_resp),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/layers/google.python.pip/pip/lib/python3.12/site-packages/google/appengine/runtime/middlewares.py", line 375, in BackgroundAndShutdownMiddleware
runtime.__BeginShutdown()
File "/layers/google.python.pip/pip/lib/python3.12/site-packages/google/appengine/api/runtime/runtime.py", line 122, in __BeginShutdown
shutdown_hook()
File "/workspace/components/campaigns/push/processor/apns/send.py", line 40, in hook
apiproxy_stub_map.apiproxy.CancelApiCalls()
File "/layers/google.python.pip/pip/lib/python3.12/site-packages/google/appengine/api/apiproxy_stub_map.py", line 375, in CancelApiCalls
self.__default_stub.CancelApiCalls()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'DefaultApiStub' object has no attribute 'CancelApiCalls'
```

## Steps to Reproduce the Problem

1. Install the shutdown hook
```
try:
from google.appengine.api import runtime, apiproxy_stub_map

def hook():
logging.info("Instance is shutting down; cleaning up.")

apiproxy_stub_map.apiproxy.CancelApiCalls()

runtime.set_shutdown_hook(hook)
except ImportError:
pass
```

2. Use the app engine service and wait for it to stop.

## Specifications

- Version: 1.1.6
- Platform: Google Appengine
- Python3.12

Start with google/appengine/api/apiproxy_stub_map.py and google/appengine/api/runtime/runtime.py, following the shutdown hook into DefaultApiStub.CancelApiCalls. Reproduce the failure on Python 3.12 with the provided hook and inspect how the default stub is initialized. Done means the shutdown path no longer raises this AttributeError, or the supported replacement and required documentation are identified with regression coverage.

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