forgejo: disable users explore page

This was requested and should make it a decent bit more difficult to get
a somewhat complete list of users on this instance.

We are, however, aware of other endpoints that can be used to get to a
similar result. Those just aren't as convenient nor obvious.

https://forgejo.org/docs/latest/admin/config-cheat-sheet/#service---explore-serviceexplore

services/forgejo/default.nix

@@ -55,6 +55,10 @@ in
           DEFAULT_KEEP_EMAIL_PRIVATE = true;
         };
 
+        "service.explore" = {
+          DISABLE_USERS_PAGE = true;
+        };
+
         oauth2_client = {
           REGISTER_EMAIL_CONFIRM = false;
           ENABLE_AUTO_REGISTRATION = true;