Digital Ocean warned that an auto-upgrade to 1.19 could not be performed for various reasons, e.g. “Validating webhook with a TimeoutSeconds value greater than 29 seconds will block upgrades.” and others. I tried applying temporary, manual changes to Capsule and Nginx’s webhooks, but soon realised that the changes would make either non-functional during the upgrade, so I might as well delete them and re-deploy them after the upgrade. If you are reading this, it worked :-P

(also upgraded various build tools)