Use render template instead of render file

Render file will need the full path in order to avoid security breaches.
In this particular case, there's no need to use render file, it's ok to
use render template.

Ref: rails/rails#35688

test/rails_app/app/views/admins/sessions/new.html.erb

@@ -1,2 +1,2 @@
 Welcome to "sessions/new" view!
-<%= render file: "devise/sessions/new" %>
+<%= render template: "devise/sessions/new" %>