Remove iPhone/iPhoneSimulator platform configurations and fix iOS build workflow

[frenzibyte]

• Depends on Remove iPhone/iPhoneSimulator platform configurations osu-framework#6351 (see dependencies there)
• Depends on framework bump

Lots of things happened.

First of all is this error:

ILLINK : warning MT0079: The recommended Xcode version for Microsoft.iOS 17.5.8018 is Xcode 15.4 or later. The current Xcode version (found in /Applications/Xcode_15.2.app/Contents/Developer) is 15.2. [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]
  		
ILLink : unknown error IL7000: An error occured while executing the custom linker steps. Please review the build log for more information. [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]
ILLINK : error MT0180: This version of Microsoft.iOS requires the iOS 17.5 SDK (shipped with Xcode 15.4). Either upgrade Xcode to get the required header files or set the managed linker behaviour to Link Framework SDKs Only in your project's iOS Build Options > Linker Behavior (to try to avoid the new APIs). [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]
ILLINK : error MT2301: The linker step 'Setup' failed during processing: This version of Microsoft.iOS requires the iOS 17.5 SDK (shipped with Xcode 15.4). Either upgrade Xcode to get the required header files or set the managed linker behaviour to Link Framework SDKs Only in your project's iOS Build Options > Linker Behavior (to try to avoid the new APIs). [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]
Error: /Users/runner/.nuget/packages/microsoft.net.illink.tasks/8.0.7/build/Microsoft.NET.ILLink.targets(87,5): error NETSDK1144: Optimizing assemblies for size failed. Optimization can be disabled by setting the PublishTrimmed property to false. [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]

Which specifies that the pinned version of Xcode that we are using is too old for the latest .NET iOS workload to work with, therefore we have to remove the pinning and use latest versions.

We did that, and we also switched macOS to run to arm64, which triggered this:

/Users/runner/.dotnet/packs/Microsoft.iOS.Sdk.net8.0_17.5/17.5.8018/targets/Xamarin.Shared.Sdk.targets(1985,3): error : No matching framework found inside '/Users/runner/.nuget/packages/ppy.veldrid.spirv/1.0.15-gca6cec7843/runtimes/ios/native/veldrid-spirv.xcframework'. SupportedPlatform: '/Users/runner/.nuget/packages/ppy.veldrid.spirv/1.0.15-gca6cec7843/runtimes/ios/native/veldrid-spirv.xcframework', SupportedPlatformVariant: 'ios', SupportedArchitectures: 'simulator'. [/Users/runner/work/osu/osu/osu.iOS/osu.iOS.csproj]

This is because the new runner uses arm64, and the script does not specify a specific runtime identifier or the iPhone/iPhoneSimulator platform that we have existing in the project, therefore .NET used iossimulator-arm64 as the runtime identifier, which we were not supporting.

Rather than sticking to x64 runners, or forcing iossimulator-x64, I've decided that it's time we just update our toolchain by removing all of these no-longer-needed configurations and add support for iossimulator-arm64 (the support consists of just having ppy/veldrid-spirv#8 in place).