Fix javadoc (#5192)
Fix javadoc (#5192)

Fixes https://github.com/apache/pulsar/issues/4979

Master Issue: https://github.com/apache/pulsar/issues/4979

### Motivation

This tool script https://github.com/apache/pulsar/blob/master/site2/tools/javadoc-gen.sh throws a lot of exceptions when it is running, resulting in many interface documents not being generated correctly. I will try to replace it with mvn javadoc:javadoc

### Modifications

* Replace `javadoc` use `mvn javadoc:javadoc`


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