Add netstandard2.0

[aholeton]

Is your feature request related to a problem? Please describe.
.NET standard 1.x has transitive dependencies on nuget packages that contain vulnerabilities. Also, targeting netstandard1.0 has a fairly large dependency graph.

.NET standard 1.x is getting a bit dated at this point. Targeting netstandard2.0 will resolve the vulnerability issues while still maintaining compatibility with .NET Framework.

https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-1-0#which-net-standard-version-to-target

Describe the solution you'd like
Target netstandard2.0

[aholeton]

@Oberon00 @z1c0 Thoughts?