fix issue where HTTP2 connection could be scavenged but not disposed

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnectionPool.cs

@@ -1999,7 +1999,8 @@ static int ScavengeConnectionList<T>(List<T> list, ref List<HttpConnectionBase>?
                 if (freeIndex < list.Count)
                 {
                     // We know the connection at freeIndex is unusable, so dispose of it.
-                    toDispose ??= new List<HttpConnectionBase> { list[freeIndex] };
+                    toDispose ??= new List<HttpConnectionBase>();
+                    toDispose.Add(list[freeIndex]);
 
                     // Find the first item after the one to be removed that should be kept.
                     int current = freeIndex + 1;