Use correct object to synchronize on in DefaultChannelPipeline (#9171)

Motivation:

8e72071d76 did adjust how synchronization is done but missed to update one block and so used synchronized (this) while it should be synchronized (handlers) .

Modifications:

Use synchronized (handlers)

Result:

Correctly synchronize

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -164,7 +164,7 @@ public class DefaultChannelPipeline implements ChannelPipeline {
         DefaultChannelHandlerContext newCtx = newContext(name, handler);
         EventExecutor executor = executor();
         boolean inEventLoop = executor.inEventLoop();
-        synchronized (this) {
+        synchronized (handlers) {
             if (context(name) != null) {
                 throw new IllegalArgumentException("Duplicate handler name: " + name);
             }