Update Azure Pipelines OS & SDK

Author: devlead

azure-pipelines.yaml

@@ -1,16 +1,22 @@
 strategy:
   matrix:
     linux:
-      imageName: 'ubuntu-20.04'
+      imageName: 'ubuntu-latest'
     mac:
-      imageName: 'macos-10.15'
+      imageName: 'macos-latest'
     windows:
-      imageName: 'windows-2022'
+      imageName: 'windows-latest'
 
 pool:
   vmImage: $(imageName)
 
 steps:
+- task: UseDotNet@2
+  displayName: 'Use .NET SDK 6.0.x'
+  inputs:
+    packageType: sdk
+    version: '6.0.x'
+
 - task: UseDotNet@2
   displayName: 'Use .NET Core sdk'
   inputs: