Fix VA YAML compatibility issues caused by version mismatch

Author: matiasroldan6

samples/csharp/assistants/virtual-assistant/VirtualAssistantSample/Pipeline/VirtualAssistantSample.yml

@@ -1,31 +1,28 @@
 # specific branch build
 trigger:
- branches: 
+ branches:
  include:
  - master
  - feature/*
- 
+
 pool:
- name: Hosted VS2017
- demands:
- - msbuild
- - visualstudio
+ vmimage: 'windows-2019'
 
 variables:
  buildPlatform: 'Any CPU'
  buildConfiguration: 'Release'
 
 steps:
-- task: DotNetCoreInstaller@0
- displayName: 'Use .NET Core sdk 2.2.100'
+- task: DotNetCoreInstaller@2
+ displayName: 'Use .NET Core sdk 3.1'
  inputs:
- version: 2.2.100
+ version: 3.1.x
  continueOnError: true
 
 - task: NuGetToolInstaller@0
- displayName: 'Use NuGet 4.9.1'
+ displayName: 'Use NuGet 5.3.0'
  inputs:
- versionSpec: 4.9.1
+ versionSpec: 5.3.0
 
 - task: NuGetCommand@2
  displayName: 'NuGet restore'
@@ -68,4 +65,4 @@ steps:
  TargetFolder: '$(Build.ArtifactStagingDirectory)\VA'
 
 - task: PublishBuildArtifacts@1
- displayName: 'Artifacts'
+ displayName: 'Artifacts'