.net standard 2.1 should be strict superset

The .netstandard2.1 version replaces some methods such as selectasync on IKRouter with iasyncenumerable. This causes problems if this library is used in a .netstandard2.0 library that is then used in a .net5+ app. The app will pull the 2.1 version of this library and then at run time a missing method exception will be thrown because the library is missing the non async enumerable returning method.