docs: update CircleCI config example

[charpeni]

CircleCI deprecated legacy images circleci/* and they won't be supported anymore on December 31, 2021.

Updating this to use cimg/node instead as well as the circleci/browser-tools orb to install Chrome.

[adamdmharvey]

Good one! Thanks

[arthurboss]

CircleCI deprecated legacy images circleci/* and they won't be supported anymore on December 31, 2021.

Updating this to use cimg/node instead as well as the circleci/browser-tools orb to install Chrome.

Was about to create the same PR. Our company just faced the issue on ci in which we were missing the circleci/browser-tools orb and lighthouse wouldn't find Chrome's installation and fail, hence I landed here.

[connorjclark]

Thanks!