🔧 Bump config.load_defaults to 7.0

Now that all necessary default overrides are in place, we can bump
`config.load_defaults` for Rails 7.

config/application.rb

@@ -12,7 +12,7 @@
 module CaseflowCertification
   class Application < Rails::Application
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 6.1
+    config.load_defaults 7.0
 
     # Configuration for the application, engines, and railties goes here.
     #