deprecate net452 + generate model based on v1.20.0 (#536)

* remove most net452 related code

* first net4 remove

* migrate test proj to net5 base

* fix format

* update sta xunit to fix platform not support

* Squashed commit of the following:

commit 16e1f819058ad281e1571b356c10d4d6ce77cf38
Author: Boshi Lian <farmer1992@gmail.com>
Date:   Tue Dec 8 22:42:57 2020 -0800

    temp disable some version converter

commit 7d1a651f4e7d27d1e61c91f46f73ac8d04ea8ab9
Author: Boshi Lian <farmer1992@gmail.com>
Date:   Tue Dec 8 20:55:44 2020 -0800

    add missing watcher generator files

commit 3f3199aad269bf89406ea71d0bc63f1a7ec23245
Author: Boshi Lian <farmer1992@gmail.com>
Date:   Tue Dec 8 22:14:47 2020 +0000

    gen v1.20.0

* bump version to 4.0

* support empty spec

* fix version converter for generator

* add generated header

* fix warning

* rerun generator

tests/KubernetesClient.Tests/AuthTests.cs

@@ -165,7 +165,7 @@ namespace k8s.Tests
             }
         }
 
-#if NETCOREAPP2_1 // The functionality under test, here, is dependent on managed HTTP / WebSocket in .NET Core 2.1 or newer.
+#if NETSTANDARD2_1 // 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 +338,7 @@ namespace k8s.Tests
                 }
             }
         }
-#endif // NETCOREAPP2_1
+#endif // NETSTANDARD2_1
 
         [Fact]
         public void ExternalToken()