Replace F extension with string interpolation

OpenRA.Game/Primitives/Size.cs

@@ -66,7 +66,7 @@ namespace OpenRA.Primitives
 
 		public override string ToString()
 		{
-			return string.Format("{{Width={0}, Height={1}}}", Width, Height);
+			return $"{{Width={Width}, Height={Height}}}";
 		}
 	}
 }