CI: Update Node, and use matrix with org var to set Node version for tests workflow

[DeeDeeG]

Summary of changes

• Update from Node 16 to Node 20 in publish.yml workflow, to loosely match core repo
• Use the org variable to set Node version in tests.yml workflow

(Also, for the tests.yml workflow job matrix, keep a last known-good Node version in the matrix, so if things were ever to break due to the org variable changing, we have some paper trail of a known-good configuration.)

Alternative approaches

I left the Node version hard-coded in the publish workflow. (Albeit I did bump it from Node 16 to 20).

I didn't want to add complexity and potential friction to the publish.yml workflow. Sanity check testing could run on a matrix of multiple Node versions, I suppose... but that's really what the tests.yml workflow is for, IMO. Obviously the actual publish step should only run once.

(Most of the time, it should be the same major Node version in each matrix job anyway, if we're punctual about updating these various repos to sync to core...)

I'm open to adding a test matrix for the publish.yml workflow's first half if people really want it.

Verification

CI passes

[confused-Techie]

While personally, I'd prefer publication runs under the 'current' NodeJS version, there is value in having it happen in a known good version all the time, so not a blocker in the slightest.

This look great to me!

(Also was rather excited to see someone else add to the Repository Revitalization Board!!)