React to System.Text.Json namespace changes (#11053)

Author: sebastienros

src/Servers/Kestrel/perf/PlatformBenchmarks/BenchmarkApplication.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.IO.Pipelines;
+using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http;