Use the new method for getting secret_key_base

See https://blog.saeloun.com/2023/08/11/rails-7-1-store-secret-key-base-in-rails-config/

app/controllers/concerns/blacklight/token_based_user.rb

@@ -50,16 +50,7 @@ def export_secret_token
 
   def secret_key_generator
     @secret_key_generator ||= begin
-      app = Rails.application
-
-      secret_key_base = if app.respond_to?(:credentials)
-                          # Rails 5.2+
-                          app.credentials.secret_key_base
-                        else
-                          # Rails <= 5.1
-                          app.secrets.secret_key_base
-                        end
-      ActiveSupport::KeyGenerator.new(secret_key_base)
+      ActiveSupport::KeyGenerator.new(Rails.application.secret_key_base)
     end
   end