feat: Change supported browsers for pre-compiled version

This changes our supported browsers from IE11 to:

- Minimum Chrome / Edge 84
- Minimum Firefox 68
- Minimum Safari 13.1

This aligns our pre-compiled version of GitLab UI with the GitLab
project.

BREAKING CHANGE: Pre-compiled version of GitLab UI dropped older browser
support. If you need to use GitLab UI with older browsers, consume the
source of GitLab UI directly.