Update build env to netcore31

Author: christophwille

azure-pipelines.yml

@@ -19,6 +19,13 @@ strategy:
         buildConfiguration: 'Debug'
 
 steps:
+- task: UseDotNet@2
+  displayName: 'Install .NET Core 3.1'
+  inputs:
+    packageType: sdk
+    version: '3.1.100'
+    installationPath: $(Agent.ToolsDirectory)/dotnet
+      
 - task: NuGetToolInstaller@1
 
 - task: NuGetCommand@2

global.json

@@ -1,5 +1,5 @@
 {
   "sdk": {
-    "version": "3.0.100"
+    "version": "3.1.100"
   }
 }