stub: mark CallStreamObserver stable

I believe this was overlooked in grpc#7938's work to fix grpc#1788.

Author: benjaminp

stub/src/main/java/io/grpc/stub/CallStreamObserver.java

@@ -50,7 +50,6 @@
  * <p>DO NOT MOCK: The API is too complex to reliably mock. Use InProcessChannelBuilder to create
  * "real" RPCs suitable for testing.
  */
-@ExperimentalApi("https://github.com/grpc/grpc-java/issues/1788")
 public abstract class CallStreamObserver<V> implements StreamObserver<V> {
 
   /**