[MIGRATION] Support upgrade assistant on macOS

[ChristopherStephan]

Upgrade Assistant

The upgrade assistant is not supported on macOS.

Running upgrade-assistant upgrade <my-solution.sln> returns

This tool is not supported on non-Windows platforms due to dependencies on Visual Studio.

Is it planned to be supported in the future? Could you document that it is currently not supported on macOS?

[follesoe]

And, as a minimum, state this in the documentation on https://learn.microsoft.com/en-us/dotnet/maui/migration/upgrade-assistant?view=net-maui-7.0

Pretty annoying as I was just getting ready to give this a shot, only to be faced with that message. Having to dig up a Windows machine was not on my TODO list for today 😫