Fix javadocs build with JDK < 12

While doing the "Website update process" in the 3.1.1 release I found that I'd broken the javadoc search functionality due to having build the Java docs with Java 11. Java < 12 a bug that means the javadoc search functionality adds /undefined/ in the URL path (even though links between pages otherwise work.

We could fix the build.gradle to use -no-module-directories when running with javadoc < v12, but that will then break the links to the JDK classes javadocs from the Kafka javadoc, as described here.

Alternatively we could change the release process docs to require building with Java 17. While this would fix the problem for the Javadocs published on the website, anyone building the javadocs for themselves would still be affected.