sorbet: Make tests `typed: false`

- Now that we've enabled `--experimental-rspec-mode` [1], let's start
  using Sorbet in tests at its basic `typed: false` level for now.
- This will ease the migration to `typed: true` of any or all tests we
  want to do in the future.

[1]: https://github.com/Homebrew/brew/pull/21742