Update SYSLOG5224BASE for ECS 8.2

[fgierlinger]

In ECS 8.2.0 multiple syslog fields (such as log.syslog.version and log.syslog.structured_data) have been introduced (elastic/ecs#1793).

The current SYSLOG5424BASE pattern stores some of the syslog fileds in the system field. This field is not defined in the ECS standard.

This PR changes 2 fields of the SYSLOG5242BASE pattern, to use the newly defined fields of the ECS 8.2 definition.

• system.syslog.version -> log.syslog.version
• system.syslog.structured_data -> log.syslog.structured_data

The other fields of the SYSLOG5242BASE have been left unchanged, because changing host.hostname to log.syslog.hostname) would introduce a breaking change. I can image many pipelines rely on the host.hostname field.