Updating to dotnet 5.0 (#1218)

* adding network file validator contract

* updating to dotnet 5

* reverting change in pipeline

* updating target framework to 5.0

* updating srgen and using target framework var

* reverting srgen back to 3.1

* Adding dotnet 3.1 to build

* Fixing task def

* removing global json

* updating other 3.1 refs to 5.0

* removing netcore folder from build.cake

* Moving and renaming dotnet 3.1 task

global.json

@@ -1,5 +1,5 @@
 {
     "sdk": {
-        "version": "3.1.406"
+        "version": "5.0.301"
     }
 }