Improve Javadoc generation

[Marcono1234]

Changes

• Use JDK 16 for creating the Javadoc to get the latest features, such as a search bar; the code will still be compiled with JDK 8
• Hide internal impl package from generated Javadoc (see Hide impl package from javadoc (and module declaration) #487)

References

Fixes #489
Resolves partially #487

Testing

Open the generated Javadoc in the browser:

• There should be a search bar in the top right corner
• The impl package (and its classes) should not be documentation

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors

[jimmyjames]

🚀 nice!