Clear cache dialog should default to  "clear" [regression]

# Summary

Via @tvst :

> The "clear cache" dialog used to start with the "Clear cache" button focused. This way you can press "C" then "Enter" to clear the cache. This seems to have regressed (maybe when we migrated dialogs to Baseweb?)

# Steps to reproduce

## Is this a regression?

yes 

# Debug info

- Streamlit version: 0.75-special
