When we have upgraded the Jenkins master from Debian Stretch to Debian Buster, we have kept using Java 8 notably because of the Gearman plugin on the CI ones. The releases one can be switched though.
The Jenkins agents are already running using Java 11.
A prerequisite for CI is to upgrade the Gearman plugin. There is a fork of the OpenDev plugin somewhere on Github and it will need to be forked on our Gerrit and re build.
- New Gearman plugin