* [dotnet] Show an error if an app developer tries to publish a simulator architecture.
* We can't publish a simulator build, so show an error in that case.
* We can't change the default runtime identifier when publishing (to a
publishable runtime identifier), because by the time we know we're
publishing, it's too late to change the runtime identifier. This means that
it'll be required for app developers to specify a runtime identifier when
publishing to a mobile platform, since the current default runtime
identifier is for a simulator build.
Partial fix for https://github.com/xamarin/xamarin-macios/issues/12997.
* Fix typo and improve naming.
* [dotnet] Don't use '_UsingDefaultRuntimeIdentifier', it's already used elsewhere in .NET