[net9.0] Add a dependency on the xcode16 packages.

[rolfbjarne]

This means it will be possible to use our Xcode 16 preview packages when
building projects by using the corresponding OS version in the TargetFramework:

<PropertyGroup>
	<TargetFramework>net8.0-ios18.0</TargetFramework>
	<NoWarn>$(NoWarn);XOCDE_16_0_PREVIEW</NoWarn>
</PropertyGroup>

Fixes #18343.

[rolfbjarne]

So EnablePreviewFeatures=true doesn't quite work, because it requires the building .NET and the target .NET to be on the same version. We're building with .NET 9, and targeting .NET 8, so it doesn't work.

This behavior is explained here: https://github.com/dotnet/designs/blob/main/accepted/2021/preview-features/preview-features.md#meaning-of-property-in-multi-targeted-projects

The best solution seems to switch to using the Experimental attribute instead, which was designed for our scenario (and explicitly to fix the problem we're running into): https://github.com/dotnet/designs/blob/bba3216250cb29b0063bac3ebb57a542ee21ad4f/accepted/2023/preview-apis/preview-apis.md

As an alternative, adding this to the csproj also works:

<NoWarn>$(NoWarn);CA2252</NoWarn>

I'm leaning towards shipping this in RC 1 as is (and tell people to use the NoWarn property), and then for RC 2+ use the Experimental attribute instead.

[vs-mobiletools-engineering-service2]

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• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
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• DotNet tests: Failed (Execution failed with exit code 1)

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✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
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[rolfbjarne]

This won't be necessary.