feat: Bump Node.js to version 18

[rmunn]

Is your feature request related to a problem? Please describe.
Our app Dockerfile uses Node 16, and Node 22 just came out last week. As a first step, let's bump the version of Node to 18.

Describe the solution you'd like
Hopefully everything still runs correctly with Node 18.

Describe alternatives you've considered
None. We may be falling behind on PHP versions, but we want to keep current on Node.

Additional context
We'll eventually want to go to Node 22, or at least Node 20 if 22 would require major rewrites.

[rmunn]

Closing; we chose to bump node.js to version 22 instead.