ButtonGroup: `The pseudo class ":first-child" is potentially unsafe when doing server-side rendering. Try changing it to ":first-of-type"`

[silvalaura]

Describe the bug ButtonGroup gives this error:

To Reproduce Steps to reproduce the behavior:

1. Go to the ButtonGroup styleguide or docs site
2. Open the console

Expected behavior No error or warning exists.

Additional context We'll need to rethink how we style ButtonGroup as ":first-of-type" is not equivalent to ":first-child"

[silvalaura]

This is super annoying and every one of our adopters gets this warning in their console. The downside is that it's not a straightforward fix 🙃

[silvalaura]

This is related to the version of Storybook we are on. Could potentially be addressed when we look at #1064.