fix: security checks

[boydc2014]

Task #minor

To fix the security check

• Set our ci dotnet SDK to 3.1. Our UseDotNet action actually never worked because we are using the "runtime", which only allows application to run but not build. What we need is "sdk" to be able to build the solution.
• Remove .net core 2.0 dependency, this would solve the most cases of of dependencies to asp.net core package in nuget, since starting from .net core 3.0, asp.net core is referred by "framework references" instead of the previous package references.
• Explicit set system.text.encodings.web, because even though in step 2, we removed everything in .net core 2.0, we still have one dependency "Microsoft.Bot.Builder.Integration.ApplicationInsights.Core" that have transient dependency to nuget packages.

[coveralls]

Coverage remained the same at 52.322% when pulling 4e8f10b on donglei/fix-pipeline into 231a8b9 on main.