chore: add deprecation notice to README post migration to the monorepo (#2737)

Added deprecation notice for the repository.

Author: meltsufin

README.md

@@ -1,3 +1,6 @@
+**_THIS REPOSITORY IS DEPRECATED. ALL OF ITS CONTENT AND HISTORY HAS BEEN MOVED TO [GOOGLE-CLOUD-JAVA](https://github.com/googleapis/google-cloud-java/tree/main/java-pubsub)_**
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 # Google Cloud Pub/Sub Client for Java
 
 Java idiomatic client for [Cloud Pub/Sub][product-docs].