Simplify projects

[0xced]

• Use the .NET 5 SDK (updated in global.json)
• Define the target frameworks only once, without condition on the operating system. Now that the Microsoft.NETFramework.ReferenceAssemblies package is automatically referenced by the .NET SDK, the project builds fine also on Linux and macOS.
• Remove the COREFX constant, use appropriate constants already defined by the SDK (NETSTANDARD1_6) where needed.
• Update all test dependencies to their latest versions
• Simplify the package dependencies, only include what's actually needed. Note that absolutely no package references are needed for .NET Standard 2.0 and 2.1

Dropping support for .NET Standard 1.6 will be easy:

1. Search for the three occurrences of #if NETSTANDARD1_6 and delete the code
2. Remove netstandard1.6 in the target frameworks
3. Remove the whole <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard1.6' ">

[mariotoffia]

Again, thanks @0xced !!! Very nice! :)