[dotnet] Fix the supported target platform versions we support.

[rolfbjarne]

FIXME: need to do something about the *_OR_GREATER symbols.

---

We've used to ignore the target platform version (the "17.0" part in "net8.0-ios17.0")
since our initial .NET relaese - customers could specify any valid OS version between
the minimum and maximum versions, and we'd completely ignore the value [1].

The purpose of the target platform version is to specify which bindings to choose:
"net8.0-ios17.0" would mean that the developer wants packages that have bindings
for iOS 17.0 (and earlier iOS versions, but not later iOS versions).

So saying "net8.0-ios11.0" would technically mean that the developer would want our
bindings for iOS 11.0 (and earlier iOS versions, but not later iOS versions). The
problem is that we don't ship any such thing... we shipped iOS 17.0 bindings in .NET
8, and that's it, you can't choose to build with something that does not have bindings
for iOS 17.0.

This will change with multi-targeting: we'll support some matrix of bindings. For
instance, we might want to support the OS version we shipped initial support in any
given .NET release + the latest OS version.

For example, we might want to support both of these:

• net8.0-ios17.0
• net8.0-ios17.2

This means that the target platform version (17.0/17.2) can't keep staying ignored.

The proper way to do this is to change the SdkSupportedTargetPlatformVersion item
group to only include target platform version we actually support, but the downside
of this approach is that it will make existing projects fail to compile, if they
happened to include an invalid target platform version.

So I've added a compatibility mode for .NET 8, where we show a warning (and tell
the developer what to do) instead of an error, and then in .NET 9 we'll make the
warning an error instead. This is accomplished by still lying in the SdkSupportedTargetPlatformVersion
item group in .NET 8, and detect the condition in our own logic and report the error.
For .NET 9 we can remove this logic, makes ure SdkSupportedTargetPlatformVersion
is correct, and then .NET itself will show the proper error.

Side note: Android is also making an invalid target platform version an error in
.NET 9: dotnet/android#8569.

[1]: We'd ignore the value for executable projects. It did have an effect for library
projects that were packed into NuGets: the target platform version would be stored
in the NuGet.

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• xamarin.ios-17.1.0.336.pkg
• xamarin.mac-9.1.0.336.pkg

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✅ API diff for current PR / commit

Legacy Xamarin (No breaking changes)

• iOS (no change detected)
• tvOS (no change detected)
• watchOS (no change detected)
• macOS (no change detected)

NET (empty diffs)

• iOS: (empty diff detected)
• tvOS: (empty diff detected)
• MacCatalyst: (empty diff detected)
• macOS: (empty diff detected)

✅ API diff vs stable

Legacy Xamarin (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• watchOS: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)

.NET (No breaking changes)

• iOS: vsdrops gist (No breaking changes)
• tvOS: vsdrops gist (No breaking changes)
• MacCatalyst: vsdrops gist (No breaking changes)
• macOS: vsdrops gist (No breaking changes)
• Microsoft.iOS vs Microsoft.MacCatalyst: vsdrops gist

Legacy Xamarin (stable) vs .NET

• iOS: vsdrops gist
• tvOS: vsdrops gist
• macOS: vsdrops gist

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