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The sample files seem to be out of date, they have not been update to include the change from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 {`

首先定位 AdMob 範例檔案以及 common_main.cc 中顯示的 firebase::admob 參照，然後檢查目前的 Google Mobile Ads C++ SDK API。當受影響的範例不再使用已棄用的命名空間，並且能針對回報的 Android 目標成功建置時，工作就完成了。

由索引模型根據 Issue 內容生成。