Sample still using Deprecated Firebase AdMob firebase::admob

### [REQUIRED] Please fill in the following fields:

* Which Firebase Sample: Admob(Auth, Database, etc.)
* Firebase C++ SDK version: firebase_cpp_sdk_11.6.0
* Additional SDKs you are using: Admob (Facebook, AdMob, etc.)
* Platform you are using the SDK on: Windows (Mac, Windows, or Linux)
* Platform you are targeting: Android(iOS, Android, and/or desktop)

### [REQUIRED] Please describe the issue here:

The sample files seem to be out of date, they have not been update to include the change from
recated Firebase AdMob C++ SDK"firebase::admob" to New Google Mobile Ads C++ SDK

(Please list the full steps to reproduce the issue. Include device logs, and stack traces if available.)

#### Steps to reproduce:

What's the issue repro rate? (eg 100%, 1/5 etc)

What happened? How can we make the problem occur?
This could be a description, log/console output, etc.

Download and following information in sample to setup and run a sample code

If you have a downloadable sample project that reproduces the bug you're reporting, you will
likely receive a faster response on your issue.

```
// A simple listener that logs changes to a BannerView.
class LoggingBannerViewListener : public firebase::admob::BannerView::Listener {
public:
LoggingBannerViewListener() {}
void OnPresentationStateChanged(
firebase::admob::BannerView* banner_view,
firebase::admob::BannerView::PresentationState state) override {
::LogMessage("BannerView PresentationState has changed to %d.", state);
}
void OnBoundingBoxChanged(firebase::admob::BannerView* banner_view,
firebase::admob::BoundingBox box) override {
::LogMessage(
"BannerView BoundingBox has changed to (x: %d, y: %d, width: %d, "
"height %d).",
box.x, box.y, box.width, box.height);
}
};`// A simple listener that logs changes to a BannerView.
class LoggingBannerViewListener : public firebase::admob::BannerView::Listener {
public:
LoggingBannerViewListener() {}
void OnPresentationStateChanged(
firebase::admob::BannerView* banner_view,
firebase::admob::BannerView::PresentationState state) override {
::LogMessage("BannerView PresentationState has changed to %d.", state);
}
void OnBoundingBoxChanged(firebase::admob::BannerView* banner_view,
firebase::admob::BoundingBox box) override {
::LogMessage(
"BannerView BoundingBox has changed to (x: %d, y: %d, width: %d, "
"height %d).",
box.x, box.y, box.width, box.height);
}
};
```

E:/code2023/123456/testapp/src/common_main.cc:26:52: error: no member named 'admob' in namespace 'firebase'
class LoggingBannerViewListener : public firebase::admob::BannerView::Listener {`

Start by locating the AdMob sample files and the firebase::admob references shown in common_main.cc, then check the current Google Mobile Ads C++ SDK API. The work is done when the affected samples no longer use the deprecated namespace and build successfully for the reported Android target.

Written by the indexing model from the issue text.