Fix javadoc generation

The upgrade to the latest javadoc maven plugin (version 3.0.0 coming from parent 26+) basically breaks our build, so I had to revert it (see https://github.com/resteasy/Resteasy/commit/4ee75f6a9a6ccf41a20adfd8fa0d5ed3279995b3 ).
We should fix the javadoc in the code so that the latest version of the plugin is able to process it.

Moreover, recent plugins do not properly aggregate the javadoc output coming from multiple modules, so basically mvn javadoc:aggregate command is also needed. This has to either be fixed or documented in the release procedure documentation.