emsdk

Upgrade node from v16.20.0 to v18.20.3 (#1387)

Node v18 is that current LTS release of node and v18.20.3 is the latest
release of v18.

This change means that emsdk is no longer installable on Ubuntu/Bionic
18.04, and we now require Ubuntu/Focal 20.04.

See: #1183
Fixes: #1173