Align priority levels for WriteEndStream and WriteTrailers

The priority levels for scheduling `WriteEndStream` and
`WriteTrailers` in the `ChannelsPriorityResponseWriter` class
have been adjusted to match the priority level of `WriteData`
(+20). Previously, `WriteEndStream` and `WriteTrailers` had
higher priority levels (+60 and +40, respectively). This change
ensures these operations are written after all `DATA` but with
the same priority. Comments were added to clarify this behavior.

Author: ladeak

src/CHttpServer/CHttpServer/ChannelsPriorityResponseWriter.cs

@@ -105,10 +105,10 @@ public void ScheduleWriteData(Http2Stream source) =>
         _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteData, GetPriorityLevel(source) + 20));
 
     public void ScheduleEndStream(Http2Stream source) =>
-        _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteEndStream, GetPriorityLevel(source) + 60));
+        _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteEndStream, GetPriorityLevel(source) + 20)); // Written after all DATA, can use the same priority
 
     public void ScheduleWriteTrailers(Http2Stream source) =>
-        _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteTrailers, GetPriorityLevel(source) + 40));
+        _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteTrailers, GetPriorityLevel(source) + 20)); // Written after all DATA, can use the same priority
 
     public void ScheduleWriteWindowUpdate(Http2Stream source, uint size) =>
         _channel.Writer.TryWrite(new ChannelStreamWriteRequest(source, WriteWindowUpdate, 1, size));