DefaultHttp2ConnectionEncoder async SETTINGS ACK SimpleChannelPromiseAggregator promise usage

Motivaiton:
DefaultHttp2ConnectionEncoder uses SimpleChannelPromiseAggregator to combine two
operations into a single future status. However it directly uses the
SimpleChannelPromiseAggregator object instead of using the newPromise() method
in one case. This may result in premature completion of the aggregated future.

Modifications:
- DefaultHttp2ConnectionEncoder to use
  SimpleChannelPromiseAggregator#newPromise() instead of directly using the
SimpleChannelPromiseAggregator instance when writing the settings ACK frame

Result:
More correct status for the SETTING ACK frame writing when auto settings ACK is
disabled.

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2ConnectionEncoder.java

@@ -291,7 +291,7 @@ public class DefaultHttp2ConnectionEncoder implements Http2ConnectionEncoder, Ht
         // Acknowledge receipt of the settings. We should do this before we process the settings to ensure our
         // remote peer applies these settings before any subsequent frames that we may send which depend upon
         // these new settings. See https://github.com/netty/netty/issues/6520.
-        frameWriter.writeSettingsAck(ctx, aggregator);
+        frameWriter.writeSettingsAck(ctx, aggregator.newPromise());
 
         // We create a "new promise" to make sure that status from both the write and the application are taken into
         // account independently.