gnu: widelands: Ensure Guix catches versioning scheme switch.

* gnu/packages/games.scm (widelands-21): New variable.

"21" was the last beta release.  Upstream has now switched to a new versioning
scheme, with lesser numbers.

gnu/packages/games.scm

@@ -5303,6 +5303,12 @@ (define-public widelands
                    license:silofl1.1       ;Widelands.ttf
                    license:cc-by-sa3.0)))) ;some music files
 
+(define-public widelands-21
+  (package
+    (inherit widelands)
+    (version "21")
+    (properties `((superseded . ,widelands)))))
+
 (define-public starfighter
   (package
     (name "starfighter")