When we [recently updated][0] our supported browser versions, we dropped
support for Safari < 13.0. Our docs state that we could drop support for
Safari 13.0 as well, while still supporting 13.1.

Safari 13.0 is barely used (and End of Life anyway) and dropping support
for it will allow us to remove polyfills for the [ResizeObserver API][1]
andto use the newer [Clipboard APIs][2].

[0]: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63994
[1]: https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver
[2]: https://developer.mozilla.org/en-US/docs/Web/API/Clipboard

Changelog: changed

This increases supported browser versions to:

- Google Chrome/Chromium: at least major version 84 because older
  versions have an annoying bug with Array.reduce:
  https://bugs.chromium.org/p/chromium/issues/detail?id=1049982
- Microsoft Edge: at least majoy version 84 (dropping support for
  pre-Chromium Edge). (Same reasoning as with Chrome)
- Mozilla Firefox: at least major version 78, as this is the latest ESR
  and it is used quite a lot.
- Apple Safari: at least major version 13 (dropping support for
  Safari 12) because it is the second latest version of Safari

Changelog: changed

In order to re-use our browserslist config across babel, autoprefixer
and other tools, we are moving it to a separate file.

Furthermore we update browserslist itself, because why not.