gollum: Transfer maintainership to erictapen

I'm not using Gollum anymore while Justin still uses it and has also
written the NixOS module. So it makes perfect sense to pass it on :)

nixos/modules/services/misc/gollum.nix

@@ -115,4 +115,6 @@ in
       };
     };
   };
+
+  meta.maintainers = with lib.maintainers; [ erictapen ];
 }