Multitarget .NET Standard 2.1

[twsl]

Since v5 is not released as an official nuget package yet I'm pushing this minor update to the v4 repo instead.

Contains:

• Added netstandard2.1 to TargetFrameworks

Additional:

• Make unit test language invariant

In order to test my changes I ran the unit tests and noticed a language depending unit test. Instead it is now based on the stack trace which makes this language invariant.

Fixes #1041

I'm not too sure if this minor fix actually requires an entry to the changelog. If you want me to add one, please say so.

[stakx]

@twsl, I'd like to merge this, but it might be better to wait until Castle.Core has a netstandard2.1 target, too. So this could remain open for a while.

But we could fix that language-dependent test now. Would you be willing to submit that second commit as a separate PR and remove it from this one?

[stakx]

@twsl I've merged this even though there still isn't a DynamicProxy release that will directly support .NET Standard 2.1. Turns out this is still useful in order to implement a workaround for #972.

Thank you for contributing!