Need newer node.js on Ubuntu 18.04 for WebEngine 5.15.3

QtWebEngine 5.15.3 is scheduled for its final update of Chromium to be based on Chrome 87. This versions makes node.js mandatory for building the devtools, and we haven't found a way to make it optional again. 

Also it appears the node.js version 8.10 installed on Ubuntu 18.04 doesn't compile correctly. Though it is inconsistent, see http://coin.intra.qt.io/coin/integration/qt/qtwebengine/tasks/web_qt_qtwebengine_1607619254030 with some platforms managing to build but not link, while others seem to not be able to parse "async import".

I believe the simplest solution would be to use a more modern node.js version, either by downloading and installing official packages like we do for Windows, or by using a PPA like https://deb.nodesource.com/node_14.x/.

Note SLES also is missing node.js, and at least downloading packages could also work for it.