Turns out class version 55 = JDK version 11. After installing JDK 13 (class version 57), Eclipse showed that the active library for the Mendix project was 13.  I installed JDK 11, right-clicked “JRE System Library” (in Eclipse, in the Project/Package explorer window) and changed it to 11.  I then restarted Mendix, selected Project->Clean Deployment Directory, and then Project->Deploy for Eclipse and that fixed it.