Revisit node version policy and practice

[bickelj]

In #355 we made the node major version explicit to both humans and machines to the point where we cannot compile or run with the "Current" node version but can with the "Active LTS" version per our declared intent. The goal was to make dependabot better at reporting @types/node changes and also make it clearer which version we expect to work.

An API user recently noted that the software no longer compiled or ran locally. It is probably due to that change. I have suggested the user try node 18.

Given that we eventually would like to use a matrix testing strategy to flag any issues with upcoming node versions, given that a user has reported an issue, and given the gap in the node Active LTS calendar where from 2023-10-18 through 2023-10-24 there will be no "Active LTS" release, I think it's worth revisiting our policy.

I move that instead of targeting the "Active LTS" version, that "we may target the 'Active LTS' or soon-to-become 'Active LTS' version where 'soon-to-become' means 'an even-numbered Current major version planned to become Active LTS within six months.'"

[slifty]

That sounds good to me -- I would suggest the following:

1. We still enforce engine requirements (e.g. that way if someone tries to run in an unsupported / untested node version they are told they must use the correct node version)
2. We update our stated policy as you suggest
3. We update the "engines" section of package.json to include Node 20 in adherence to the updated policy

[slifty]

We never moved to 20 but should -- made an issue for it #675