nixos/mediawiki: don't assume language of main page

The mainpage might be named Hauptseite and mediawiki redirects from
/wiki/ on its own to there

nixos/modules/services/web-apps/mediawiki.nix

···
 
           # Explicit access to the root website, redirect to main page (adapt as needed)
           "= /".extraConfig = ''
-            return 301 /wiki/Main_Page;
+            return 301 /wiki/;
           '';
 
           # Every other entry point will be disallowed.