Build both pnpm and yarn images

[davisagli]

What I'm trying to do here (not yet tested):

1. Rename the existing yarn-based images to be released with a -yarn suffix on the version (e.g. plone/plone-frontend:18.0.0-alpha.42-yarn`). Also update the prod image to use Node 20.
2. Update the images that are released without a suffix to use the new approach from First implementation kitconcept/plone-frontend#1, using pnpm and cookieplone instead of the old generator.
3. Remove the nightly image (it was already disabled, and would take some work to update).

[sneridagh]

LGTM