[#3431] Remove dotnet core 2.1 templates from vsix - Update templates

[ceciliaavila]

Fixes # 3431

Description

This PR updates the dotnet templates removing the netcore2.1 and netcore2.2 options for creating bots. Also, it updates the version of Microsoft.VSSDK.BuildTools to a stable version in the BotBuilderTemplatesVSIX project.

Detailed Changes

• Updated the following templates removing netcore2.1 and netcore2.2:

  • Microsoft.BotFramework.CSharp.CoreBot
  • Microsoft.BotFramework.CSharp.EchoBot
  • Microsoft.BotFramework.CSharp.EmptyBot

• Updated the version of Microsoft.VSSDK.BuildTools to 17.0.5232 in the BotBuilderTemplatesVSIX.csproj and packages.config

Testing

These images show the VSIX working in VisualStudio 2019 and VisualStudio 202, and the before and after of the dotnet templates' menu.

[ceciliaavila]

Promoted to MS in PR# 3619