Fix JavaDoc warning

[ckittl]

Currently, JavaDoc cannot be built properly with Java 17. Resolve all the issues! With #493 the gradle task is configured in a way to not fail on errors in JavaDoc. Please roll that back!

[sebastian-peter]

As this issue has been lying around for some time, please check in what way this is still relevant today.
Mabye it'd be worthwile looking into https://javadoc.io/ too.