Rollback iOS workload to last known working version

[smoogipoo]

This is required to get osu! running on iOS again.

Out of...

• .NET SDK versions 8.0.302 (last known working) and 8.0.400 (latest).
• Xcode versions 15.4 (last known working) and 16.0 (latest).
• iOS SDK/manifest versions 17.2.8053 (last known working) and 18.0.8303 (latest)

... I've isolated the problematic component to be the iOS SDK.

The version 17.2.8053 is a well-informed estimate based on release dates respective to the last working iOS release (2024.625.2) and can be cross-referenced with the manifest's nuget package: https://www.nuget.org/packages/Microsoft.NET.Sdk.iOS.Manifest-8.0.100/17.2.8053

• My assumption is that the following manifest version 17.2.8078 does not work. This is the only version that was pushed in-between two of our releases, the last-known working one and the first-known broken one. I am also not having a fun experience compiling with this SDK - it's likely the version where Microsoft broke AOT deduplication and many other things along with it.
• While the rolled-back SDK works with Xcode 16, it also works with 15.4, and I've also had better experience compiling with 15.4 (compile-time-wise). So I've reverted it. It's good to note, though, as Xcode 16 will be the only available option when GHA eventually upgrades to macOS-15.
• .NET SDK 8.0.400 added workload sets that would simplify this by allowing us to store it in global.json. Unfortunately the earliest available workload set uses the 17.2.8078 manifest, which is the first version that does not work for us.
• We will still need to isolate the issue further and report to Microsoft. I provided some initial commentary here. It might be worthwhile to check this prior to .NET 9's release (and also check if it's fixed with .NET 9).

I have tested an iOS build with full auth + score submission.