[iOS 9.0.50] MauiIcon's Include parameter must be named appicon

### Description

It looks like MauiIcon works on iOS only if it is named appicon . If I use another name like appiconios, the iOS build fails. More than one possible name would be useful. ForegroundFile works with any file name.

### Steps to Reproduce

1. Create a default MAUI app
2. Change its appicon.svg to be named appiconios.svg
3. In project file, update MauiIcon's Include to point to this file
4. Build for iOS
5. The build fails

### Link to public reproduction project repository

_No response_

### Version with bug

9.0.50 SR5

### Is this a regression from previous behavior?

Yes, this used to work in Xamarin.Forms

### Last version that worked well

Unknown/Other

### Affected platforms

iOS

### Affected platform versions

iOS 18.4.1

### Did you find any workaround?

To create another icon file used on iOS, I can make a subdirectory under AppIcon (e.g. called iOS) with another file named appicon, and then the build works. But it is all quite silly that you cannot change the file name.

### Relevant log output

```shell

```

Start with the project file's MauiIcon Include entry and the iOS build path described in the reproduction steps; compare behavior when appicon.svg is renamed to appiconios.svg. Done means an iOS build accepts a MauiIcon with a non-appicon filename and supports more than one valid name.

Written by the indexing model from the issue text.