onnxruntime

Enable C# .NET5 for WinML  (#6120)

* build for .net5

* only reference cswinrt for .net5

* remove netstandard2.0 references

* upgrade language version

* net5

* remove extra comment closure

* add targetframework

* set target framework

* remove net*

* pep8 errors

* make test project build with .net windows SDK projection

* disable c# builds for non-x64 builds

* fix pep8 errors

* disable for store build

* fix tests

* remove cswinrt and sdk references from package

* bump cswinrt down to 1.0.1

* fix bin path

Co-authored-by: Sheil Kumar <sheilk@microsoft.com>