(build) update to .net core 3.1 sdk

Author: arturcic

build.config

@@ -1,3 +1,3 @@
 #!/usr/bin/env bash
 CAKE_VERSION=0.35
-DOTNET_VERSION=2.1.802,3.0.100
+DOTNET_VERSION=2.1.802,3.1.100

build/stages/common-steps.yml

@@ -10,10 +10,10 @@ steps:
     packageType: sdk
     version: 2.1.802
 - task: UseDotNet@2
-  displayName: 'Use .NET Core sdk 3.0'
+  displayName: 'Use .NET Core sdk 3.1'
   inputs:
     packageType: sdk
-    version: 3.0.100
+    version: 3.1.100
 - task: Bash@3
   displayName: 'Env Vars'
   inputs: