Throw away everything and rebuild your api from scratch, it may be worth it for performance anyway.
I am not going to provide a full list for what to change, you will have to scour the docs and examples for what replaced your code with something new, rjweb-server 9 is a complete rewrite and almost everything except basic routing and http contexts changed completely, you can always stay on rjweb-server 8 too, however I can not guarantee security updates for an eternity.