gitea: please use postgresql 15

whoops, forgot this one. why do modules bundle other modules, anyways?

services/gitea.nix

@@ -67,6 +67,8 @@
           session.COOKIE_SECURE = true;
         };
       };
+      services.postgresql.package = pkgs.postgresql_15;
+
       services.postgresqlBackup = {
         enable = true;
         databases = [ "gitea" ];