Stop supporting multiple targets in XHarness Helix SDK

[premun]

We never supported this really and it was deprecated in dotnet/xharness#618

This will be a breaking change in the SDK:

<Targets>ios-simulator-64</Targets>

will change to

<TestTarget>ios-simulator-64</TestTarget>

[premun]

@akoeplinger we could make it a non-breaking change but I assume it will be trivial to change in the dependency update PR?

What do you think?

[akoeplinger]

@premun should be fine. Did you talk to Manuel about removing the multi-target option from xharness? Not sure if they used it on their end.

[premun]

@akoeplinger they are actually only using the Microsoft.DotNet.XHarness.iOS.Shared nuget and have their own sort of CLI on top of it:
https://github.com/xamarin/xamarin-macios/tree/main/tests/xharness