fix(ops/modules/forgejo): disable native sign-in

We only use the OAuth flow (with Keycloak), and the native login
mechanism is an unnecessary source of user confusion.

Change-Id: I819e0b6ac507013c903c55a28f0db52e8706d8dc
Reviewed-on: https://cl.snix.dev/c/snix/+/30282
Tested-by: besadii
Reviewed-by: Ryan Lahfa <masterancpp@gmail.com>
Reviewed-by: Florian Klink <flokli@flokli.de>
Autosubmit: edef . <edef@edef.eu>

ops/modules/forgejo.nix

@@ -166,6 +166,7 @@ in
           # unambiguously.
           DISABLE_REGISTRATION = false;
           ALLOW_ONLY_EXTERNAL_REGISTRATION = true;
+          ENABLE_INTERNAL_SIGNIN = false;
 
           #REQUIRE_SIGNIN_VIEW = false;
           ENABLE_NOTIFY_MAIL = true;