Update Node version from 16 to 18

[willdurand]

Node 16 is no longer supported as per the release schedule: https://nodejs.org/en/about/previous-releases. Tools such as addons-linter and web-ext no longer support Node < 18 officially, but today we were forced to fix the linter to let it run in the add-ons pipeline (see: https://github.com/mozilla/addons-linter/releases/tag/6.31.1).

We should update the Node version to v18 at least, ideally everywhere it's used so that we are consistent.