Set http 2.0 for .net 5.0+ (#833)

Author: zhiweiv

src/KubernetesClient/Kubernetes.cs

@@ -104,7 +104,7 @@ protected override HttpRequestMessage CreateRequest(string relativeUri, HttpMeth
             var httpRequest = new HttpRequestMessage();
             httpRequest.Method = method;
             httpRequest.RequestUri = new Uri(BaseUri, relativeUri);
-#if NETSTANDARD2_1_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NET5_0_OR_GREATER
             httpRequest.Version = HttpVersion.Version20;
 #endif
             // Set Headers