Enable log.format.json.fix_duplicate_message_fields by default

In #15969, we introduced a new opt-in flag `log.format.json.fix_duplicate_message_fields`, to avoid collision of field names in log lines when `log.format` is JSON. We should make this the default behaviour in `9.0`

- [x] For `9.x`, update the default value of `log.format.json.fix_duplicate_message_fields` to `true`
