Extremely confusing AppDomain documentation. Outdated?

### Type of issue

Outdated article

### Description

Even though .NET 8 is selected as the version, the page seems to contain information on how to use AppDomain on .NET Framework. For instance, this is not true in .NET 8 (and hasn't been true since .NET Core):

> Multiple application domains can run in a single process

Also, the example code might work on .NET Framework, but not on .NET 8. There's no public constructor for `AppDomainSetup`, so the code doesn't even compile.

It looks like the content hasn't been updated at all, except that some members are shown as obsolete. There's a warning about .NET Core, but it's not clear at all from the docs that the whole thing doesn't work on .NET 8.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.appdomain?view=net-8.0

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System/AppDomain.xml

### Document Version Independent Id

eae62dad-109f-0239-95c1-2777c3117163

### Article author

@dotnet-bot