Add a caution note to README about the Google Mobile Ads SDK being in maintenance mode.

PiperOrigin-RevId: 937529529

Author: malandr2

README.md

@@ -1,5 +1,8 @@
 # Google Mobile Ads SDK for Android
 
+> [!CAUTION]
+> The Google Mobile Ads SDK (play) is in maintenance mode. Use the [GMA Next-Gen SDK](https://github.com/googleads/gma-next-gen-sdk-android-examples).
+
 [![Build Status](https://github.com/googleads/googleads-mobile-android-examples/actions/workflows/build.yml/badge.svg)](https://github.com/googleads/googleads-mobile-android-examples/actions/workflows/build.yml)
 
 The Google Mobile Ads SDK is the latest generation in Google mobile advertising,