[deploys] Netlify build image needs an update

[isms]

Looks like an easy fix, but for now this is the message:

Support for the Xenial build image ended on November 15, 2022. To enable builds for this site, select another build image. Read the migration guide.

[jayqi]

I suspect this is not actually relevant. We build the site in GitHub Actions and push the built assets to Netlify, rather than having Netlify run the build. It may be the case that we have a vestigial build process set up on Netlify that we don't actually use and is disabled, and that's what Netlify is complaining about.

[pjbull]

I "unlinked" the repository, and confirmed that publishing with the API from GitHub actions still works. Also "updated" the build image JIC.