util: MultiChildLoadBalance.shutdown() log to FINE (#10935)

The log level of this method is set to INFO, which is too spammy. Bring
it down to the FINE level.

Co-authored-by: Terry Wilson <terrymwilson@gmail.com>

util/src/main/java/io/grpc/util/MultiChildLoadBalancer.java

@@ -297,7 +297,7 @@ protected boolean reactivateChildOnReuse() {
 
   @Override
   public void shutdown() {
-    logger.log(Level.INFO, "Shutdown");
+    logger.log(Level.FINE, "Shutdown");
     for (ChildLbState state : childLbStates.values()) {
       state.shutdown();
     }