Add breaking changes for log.format.json.fix_duplicate_message_fields defaults

[robbavey]

#16578 introduced a change to 9.0 to default to setting log.format.json.fix_duplicate_message_fields to true.

The flag to allow this was introduced in #15969, and provides the ability to handle incoming events that have multiple message fields by renaming the second to message_1

This is a breaking change, as it may change the expected value of the message field