Update Rails default version to 7.0 and enable Zeitwerk (#167)

Author: lionel-k

config/application.rb

@@ -9,7 +9,9 @@
 module BurundiarxivApi
   class Application < Rails::Application
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 6.0
+    config.load_defaults 7.0
+    config.active_support.disable_to_s_conversion = false
+    config.autoloader = :zeitwerk
 
     # Configuration for the application, engines, and railties goes here.
     #