docs: Node.js version requirement to be updated

Doc says as follows, which should be updated:

Make sure you have Node 8.x (LTS) and Yarn 1.2 or newer.

• https://www.npmjs.com/package/@gitlab/ui
• https://gitlab.com/gitlab-org/gitlab-ui#quick-start-development

---

cf. !713 (merged) (moved to the official node image), gitlab-build-images!199 (merged) (Node.js was upgraded from 10 to 12) and gitlab-build-images!112 (merged) (Node.js was 10 even at the inception)