Upgrade Rails defaults to 7.0

This fixes a deprecation warning and follows the linked guidance:

```
DEPRECATION WARNING: Support for `config.active_support.cache_format_version = 6.1` has been deprecated and will be removed in Rails 7.2.

Check the Rails upgrade guide at https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#new-activesupport-cache-serialization-format
for more information on how to upgrade.
```

config/application.rb

@@ -23,7 +23,7 @@ class Application < Rails::Application
     end
 
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 6.0
+    config.load_defaults 7.0
 
     # Configuration for the application, engines, and railties goes here.
     #