Official support of Node.js 14.x

Node.js 14.x, released on 2020-04-21, was marked as LTS on 2020-10-27, so gitlab-ui has to support that version of Node.js.

cf.

• https://nodejs.org/ja/blog/release/v14.15.0/
• !1689 (merged) (#975 (closed))
• gitlab!43808 (merged) (gitlab#258963 (closed) gitlab#258964 (closed))