Remove Virtus

https://github.com/solnic/virtus has been discontinued and has a warning on Ruby 3.4 that we will need to add `ostruct` if we continue to run it in Ruby 3.5.

```
/bundle/ruby/3.4.0/gems/slayer-0.5.4/lib/slayer/command.rb:14: warning: Skipping set of ruby2_keywords flag for call (method accepts keywords or post arguments or method does not accept argument splat)
/bundle/ruby/3.4.0/gems/virtus-2.0.0/lib/virtus.rb:1: warning: ostruct was loaded from the standard library, but will no longer be part of the default gems starting from Ruby 3.5.0.
You can add ostruct to your Gemfile or gemspec to silence this warning.
Also please contact the author of virtus-2.0.0 to request adding ostruct into its gemspec.
```