[mono] Fix hardcoded .NET 8.0 string in iOS sample (#103117)

We always forget updating this string with new .NET releases so let's just not mention the version.

src/mono/sample/iOS/Program.cs

@@ -49,7 +49,7 @@ public static async Task Main(string[] args)
             delegate* unmanaged<void> unmanagedPtr = &OnButtonClick;
             ios_register_button_click(unmanagedPtr);
         }
-        const string msg = "Hello World!\n.NET 8.0";
+        const string msg = "Hello World!\n.NET Runtime";
         for (int i = 0; i < msg.Length; i++)
         {
             // a kind of an animation