[📚] Update to GMA Next-Gen SDK for Android

### Documentation Feedback

Hi,
Our contacts at Google indicate that we should start using the GMA Next-Gen SDK for Android applications now due to its numerous benefits.

https://developers.google.com/ad-manager/mobile-ads-sdk/android/next-gen/quick-start

Although it's in beta, they recommend its use in production environments.
Have you considered making this update?

Thanks,
Sergio.
