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

scripts/update_node.py

@@ -16,8 +16,8 @@ import subprocess
 import os
 import shutil
 
-version = '16.20.0'
-base = 'https://nodejs.org/dist/latest-v16.x/'
+version = '18.20.3'
+base = 'https://nodejs.org/dist/latest-v18.x/'
 upload_base = 'gs://webassembly/emscripten-releases-builds/deps/'
 
 suffixes = [