JavaDoc warning is blocking all pull requests

[TimvdLippe]

Starting with #2758, our JavaDoc build has been erroring because of a change on Oracle's side. There's more information in #2759, but the gist of it is that Oracle removed the element-list URL and replaced it with the package-list URL. However, the latter can only be consumed by JavaDoc from JDK 9 and above, whereas we are running JDK 8 (since our minimum supported version is still Java 8).

We have a couple of options to remedy the situation:

1. We upgrade to Java 11 and publish Mockito 5, as we update our minimum supported version
2. We employ JUnit's workaround where we download the package-list and write it to disk as element-list, after which we use -linkoffline instead of -link to consume the file
3. We use Java 11 to publish our JavaDoc, but we use Java 8 for the rest (e.g. artifact publication)
4. We use Java 11, but we still publish with minimum version Java 8

Of all these options, I think 2 is the best we have. Upgrading to Java 11 runtime (regardless of what version we decide to publish for) is still a risk, as we want our artifacts to work on Java 8. And the best way to confirm that's all working is to run on Java 8 ourselves. I don't think we should upgrade to Java 11 yet, but it's one more drop in the bucket for us dropping Java 8 support.