Tests: Load 6.1 defaults in Dummy Application

Resolve deprecation warnings like:

```
Preparing test database
DEPRECATION WARNING: Non-URL-safe CSRF tokens are deprecated. Use 6.1 defaults or above. (called from <top (required)> at /home/runner/work/turbo-rails/turbo-rails/test/dummy/config/initializers/inspect_helpers.rb:1)
DEPRECATION WARNING: Using legacy connection handling is deprecated. Please set
`legacy_connection_handling` to `false` in your application.

The new connection handling does not support `connection_handlers`
getter and setter.

Read more about how to migrate at: https://guides.rubyonrails.org/active_record_multiple_databases.html#migrate-to-the-new-connection-handling
 (called from <top (required)> at /home/runner/work/turbo-rails/turbo-rails/test/test_helper.rb:6)
```

Since our GitHub CI matrix includes `6.1`, `7.0`, and `main`, CI's tests
should run with at least the `6.1` defaults.

Author: seanpdoyle

test/dummy/config/application.rb

@@ -12,7 +12,7 @@
 module Dummy
   class Application < Rails::Application
     # Initialize configuration defaults for originally generated Rails version.
-    config.load_defaults 6.0
+    config.load_defaults 6.1
 
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration can go into files in config/initializers