add load_defaults option to example_app

[goosys]

To ensure tests run correctly on each Rails version, I think it's better to set load_defaults accordingly.

I tried setting it, but it resulted in a large number of errors. I plan to fix them gradually over time.
Once all errors are resolved, I will open a PR.

If you have any concerns or a different direction in mind, please let me know.

[goosys]

memo

• Replace form_for with form_with #2825

[goosys]

@nickcharlton

• Enabled load_defaults to be set per Rails version
• Removed configuration values set by load_defaults from config/application.rb
  • This eliminates the warnings that were appearing at runtime

  • DEPRECATION WARNING: `config.active_support.to_time_preserves_timezone` is 
    deprecated and will be removed in Rails 8.2 (called from <top (required)> at 
    /workspaces/administrate/spec/example_app/config/environment.rb:5)
    

• Fixed the issues that were causing errors