.NET version updates, removals (#5)

* Upgrade tasks version; incl MSTest SDK in tests

* Rm'd NuGet config

* Added publish NuGet pkgs script

* Added net5.0 support

* Upgrade version to 3.0

* Add build GitHub action

* Added dependabot

* Fix path for publishing pkgs

* Added .NET 6.0; dropped netstd 2.0, netcoreapp2.1; bump minor ver

* Dropped netcore 2.1; upgraded build vers

* Revert net6.0 due to unsupported github action

Author: jburnett