Use Rails 7.1 defaults

This removes the defaults file, as we now switch to using Rails 7.1
defaults.

Author: brucebolt

config/application.rb

@@ -21,7 +21,7 @@
 module HmrcManualsApi
   class Application < Rails::Application
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 7.0
+    config.load_defaults 7.1
 
     # Please, add to the `ignore` list any other `lib` subdirectories that do
     # not contain `.rb` files, or that should not be reloaded or eager loaded.