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

src/KubernetesClient/KubernetesClientConfiguration.ConfigFile.cs

@@ -357,13 +357,7 @@ namespace k8s
                                 {
                                     var expiresOn = int.Parse(config["expires-on"]);
                                     DateTimeOffset expires;
-#if NET452
-                                    var epoch = new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero);
-                                    expires
-     = epoch.AddSeconds(expiresOn);
-#else
                                     expires = DateTimeOffset.FromUnixTimeSeconds(expiresOn);
-#endif
 
                                     if (DateTimeOffset.Compare(
                                         expires,