Appdata: change license identifier

former one is valid but deprecated

[ci skip]

(cherry-picked from commit d133b51)

packaging/wesnoth.appdata.xml

@@ -4,7 +4,7 @@
   <id>wesnoth.desktop</id>
   <launchable type="desktop-id">wesnoth.desktop</launchable>
   <metadata_license>GFDL-1.3</metadata_license>
-  <project_license>GPL-2.0+</project_license>
+  <project_license> GPL-2.0-or-later</project_license>
   <name>Battle for Wesnoth</name>
   <name xml:lang="de">Battle for Wesnoth</name>
   <name xml:lang="es">La batalla por Wesnoth</name>