This change was also announced in Shortening the Let's Encrypt Chain of Trust back in July 2023.
For any system admin using Let's Encrypt, it's a good idea to subscribe to the API Announcements category so you'll get a notification when a new thread is posted and you're up to date with any upcoming change. In the end this was a preventable problem you're facing right now, as one had more than half a year worth of time for preparations.