Replace F extension with string interpolation

OpenRA.Mods.Common/Widgets/LabelWidget.cs

@@ -68,7 +68,7 @@ namespace OpenRA.Mods.Common.Widgets
 		public override void Draw()
 		{
 			if (!Game.Renderer.Fonts.TryGetValue(Font, out var font))
-				throw new ArgumentException("Requested font '{0}' was not found.".F(Font));
+				throw new ArgumentException($"Requested font '{Font}' was not found.");
 
 			var text = GetText();
 			if (text == null)