Update app dev guide for Java Driver 4.x mapper

Description

See https://datastax.jira.com/browse/DOC-2547.

Currently the Object Mappers page in the app dev guide contains info only on the 3.x version of the driver. We should update it to add some details for the new mapper added in 4.x.

See here: https://docs.datastax.com/en/devapp/doc/devapp/driversObjectMappers.html#driversObjectMappers__javaObjectMapper

Environment

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Pull Requests

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Assignee

Erik Merkle

Reporter

Alexandre Dutra

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