Fix concurrency warning by adding @MainActor annotation in protocol definition

The warning was: "Sending 'self.appIconConfigurator' risks causing data races;
this is an error in the Swift 6 language mode"

Modules/App/Sources/UI/Screens/AppSettings/AppIcon/AppIconConfigurable.swift

@@ -21,6 +21,7 @@ protocol AppIconConfigurable {
     var alternateIconName: String? { get }
     var supportsAlternateIcons: Bool { get }
 
+    @MainActor
     func setAlternateIconName(_ alternateIconName: String?) async throws
 }