Use same code for Json Platform as TE (#7292)

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

@@ -117,7 +117,7 @@ namespace PlatformBenchmarks
 
             // Content-Length header
             writer.Write(_headerContentLength);
-            var jsonPayload = JsonSerializer.ToBytes(new { message = "Hello, World!" }, SerializerOptions);
+            var jsonPayload = JsonSerializer.ToUtf8Bytes(new JsonMessage { message = "Hello, World!" }, SerializerOptions);
             writer.WriteNumeric((uint)jsonPayload.Length);
 
             // End of headers
@@ -155,5 +155,10 @@ namespace PlatformBenchmarks
             PlainText,
             Json
         }
+
+        public struct JsonMessage
+        {
+            public string message { get; set; }
+        }
     }
 }