add support for asyncapi 2.1

[JanEggers]

https://www.asyncapi.com/blog/release-notes-2.1.0

i dont know if you actually need to change something in the schema or its just a matter of changing the asyncapi property of the document.

I basicly dont want to get bothered by the asyncapi ui that is telling me every time i open it that i could update

[m-wild]

I will investigate and fix. Thanks

[m-wild]

Released in https://www.nuget.org/packages/Saunter/0.7.0