default throw ServerCallStreamObserver.setOnCloseHandler (#8564)

diff --git a/stub/src/main/java/io/grpc/stub/ServerCallStreamObserver.java b/stub/src/main/java/io/grpc/stub/ServerCallStreamObserver.java
index e31cae1..8201a23 100644
--- a/stub/src/main/java/io/grpc/stub/ServerCallStreamObserver.java
+++ b/stub/src/main/java/io/grpc/stub/ServerCallStreamObserver.java
@@ -168,5 +168,7 @@
    * @param onCloseHandler to execute when the call has been closed cleanly.
    */
   @ExperimentalApi("https://github.com/grpc/grpc-java/issues/8467")
-  public abstract void setOnCloseHandler(Runnable onCloseHandler);
+  public void setOnCloseHandler(Runnable onCloseHandler) {
+    throw new UnsupportedOperationException();
+  }
 }