Update Google Chrome prerequisite

Problem

The prerequisite for Google Chrome and Chromedriver in https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/prepare.md#L20 is still version 60 and 2.33 respectively. Current version for both is 78. May be time to update the requirement.

Additionally, the docs vaguely indicate that you should visit the Chromedriver docs for more info, but don't note that installation is covered (by the included script) for the Mac installation:

Visit the Chrome Driver Getting started page for more details.

Proposal

• Determine if it's okay to update the requirement, and to what version is necessary
• Clarify that installation on Mac is covered by the install script.
• Update the link to the Chromedriver docs to: https://chromedriver.chromium.org/getting-started