superTuxKart: update license from gpl2Plus to gpl3Plus

The source code is licensed under GPLv3 or later.
This was changed upstream at Nov 23, 2018 [here](https://github.com/supertuxkart/stk-code/commit/06952bfd524ce368e12e62aadcc8546b0786051c),
but wasn't updated here in nixpkgs, until now.

The game assets are released under a mixture of licenses and noted in their respective 'licenses.txt' files.

pkgs/by-name/su/superTuxKart/package.nix

@@ -170,7 +170,7 @@ stdenv.mkDerivation rec {
       Kart.
     '';
     homepage = "https://supertuxkart.net/";
-    license = lib.licenses.gpl2Plus;
+    license = lib.licenses.gpl3Plus;
     maintainers = with lib.maintainers; [
       peterhoeg
     ];