Merge pull request #3164 from windows-toolkit/netcore31

Removed .net core version pinning, and updated the build pipeline to .Net Core 3.1

Author: azchohfi

azure-pipelines.yml

@@ -28,7 +28,7 @@ steps:
 - task: UseDotNet@2
   inputs:
     packageType: 'sdk'
-    version: '3.0.100'
+    version: '3.1.101'
   displayName: Use .NET Core sdk
 
 - task: DotNetCoreCLI@2  

global.json

@@ -1,7 +1,4 @@
 {
-  "sdk": {
-    "version": "3.0.100"
-  },
   "msbuild-sdks": {
     "MSBuild.Sdk.Extras": "2.0.54"
   }