The pdf_thumbnail plugin does not exist
I resolved this by first re-enabling the PDF module, and then by updating core.

In my composer setup, the PDF module was a git submodule.  However, my shared host would not work with that.  So, I removed the cached information

git rm --cached web/modules/contrib/pdf


And then I added the code in that directory to my main repo.

Next, enable the PDF module.

Finally, proceed with the update process, and run update.php or drush updb