Gen v1.21.0 (#603)

* gen v1.21.0

* update version converter

* bump version to 5

* remove support of netstandard2.0

* update test dependency

tests/KubernetesClient.Tests/AuthTests.cs

@@ -165,7 +165,6 @@ namespace k8s.Tests
             }
         }
 
-#if NETSTANDARD2_1 || NET5_0 // The functionality under test, here, is dependent on managed HTTP / WebSocket in .NET Core 2.1 or newer.
         // this test doesn't work on OSX and is inconsistent on windows
         [OperatingSystemDependentFact(Exclude = OperatingSystems.OSX | OperatingSystems.Windows)]
         public void Cert()
@@ -338,7 +337,6 @@ namespace k8s.Tests
                 }
             }
         }
-#endif // NETSTANDARD2_1 || NET5_0
 
         [Fact]
         public void ExternalToken()