[C#][VA] Fix Virtual Assistant's Pipeline compatibility issues

[ldardick]

Fixes #3833

Purpose

The Pipeline created using the provided VirtualAssistantSample.yml file would fail due to being outdated, as reported on issue #3833.

Changes

The following changes were applied to the Pipeline use on the sample and the VSIX template

• NuGetToolInstaller task NuGet version was updated from version 4.9.1 to 5.3.0.
• DotNetCoreInstaller task was updated to install version 3.1 of NET Core to match with the Virtual Assistant dependency.
• DotNetCoreInstaller task was upgraded to version 2.
• Build tool were upgraded for NET Core 3.1 by updating the pool version to windows-2019.

Tests

With the changes, the Pipeline is now passing:

Feature Plan

N/A

Checklist

General

• [X] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated the appropriate tests
• [ ] I have updated related documentation