onnxruntime

Add targets files for new .net6 frameworks (#12016)

* Add net6 targets.
Remove maccatalyst as we don't have a native build targetting that.

* Set platform in macos targets

* Add targetFramework entries

* Move NativeLib.DllName definition and set using preprocessor values for simplicity. Couldn't get it to build with the preprocessor based setup when it was in a separate file.

Update the nuspec generation to set platform version for .net6 targets. TODO: Validate versions. I copied them from the managed nuget package the packaging pipeline generated prior to adding targets. Possibly w could/should lower some of the versions.

Hopefully the need to specify a version goes away when the release version of VS2022 supports .net6.

* Try android 31.1 as https://github.com/actions/virtual-environments/blob/main/images/win/Windows2022-Readme.md suggests that should be available on the CI machines

* Fix patch version mismatch
Add some extra debug info in case it helps

* Debug nuget location in CI

* Add workspace entry back in

* Add steps

* One more attempt with hardcoded nuget.exe path and original android31.0 version

* Better fix - found explicit nuget download and updated version there.

* flake8 fixes

* Fix black complaints.

* Exit Microsoft_ML_OnnxRuntime_CheckPrerequisites for net6 iOS.

* Removed outdated comment