Remove .NET Standard 1.x from library

[natemcmaster]

CommandLineUtils currently compiles for .NET Standard 1.x. I'd like to remove this target so:

• Simplifies packaging requirements. .NET Standard 2.x requires managing fewer dependencies
• Unifies API - there were some APIs added to CLU that don't work in 1.x because things like System.Security.SecureString didn't exist in 1.x
• Reduce package size - one fewer .dll
• Simplify reflection code. Most of the code is written with TypeInfo b/c netstandard1.x reflection APIs were severely crippled.
• Every variant of Microsoft-supported .NET Platforms have a .NET Standard 2.0 compatible version. If you're still on an old platform that doesn't, you should update.