Use gem in Ruby 3 Ractor

I am trying to use gem in Ruby 3 Ractor. I got few exceptions like this:

```
:694:in `take': thrown by remote Ractor. (Ractor::RemoteError)
.rbenv/versions/3.0.1/lib/ruby/gems/3.0.0/gems/droplet_kit-3.15.0/lib/droplet_kit/client.rb:78:in `default_user_agent': can not access non-shareable objects in constant DropletKit::VERSION by non-main Ractor. (Ractor::IsolationError)
```

I think if you add `.freeze` on strings in the gem it will solve most of the issues

Start by inspecting lib/droplet_kit/client.rb around default_user_agent and reproduce the reported Ruby 3 Ractor::IsolationError with the gem. Trace other non-shareable constants or strings used across Ractor boundaries; done means the reported Ractor usage no longer raises the isolation error.

Written by the indexing model from the issue text.