[8.18](backport #42403) Update Journald fields to better match ECS

[mergify]

Proposed commit message

The fields produced by the Journald input are updated to better match ECS. Renamed fields:
Dropped fields: syslog.priority and syslog.facility while keeping their duplicated equivalent:
log.syslog.priority,log.syslog.facility.code. Renamed fields: syslog.identifier -> log.syslog.appname,
syslog.pid -> log.syslog.procid. container.id_truncated is dropped because the full container ID is
already present as container.id and container.log.tag is dropped because it is already present as
log.syslog.appname. The field container.partial is replaced by the tag partial_message if it was true,
otherwise no tag is added.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Because the fields produced by the Journald input are updated to better match ECS, once the user updates Filebeat/Elastic-Agent, the events generated will be slightly different.

Author's Checklist

• Check if the updated fields are used by the Elastic-Agent system integration

How to test this PR locally

Run Filebeat with the journald input:

filebeat.inputs:
- type: journald
  id: my-journald-id

Verify the changes:

Renamed fields:

• log.syslog.priority
• log.syslog.facility.code
• log.syslog.appname
• log.syslog.procid

Removed fields:

• container.id_truncated
• container.log.tag

Instead of container.partial, now we set the tag partial_message

To validate that the tag partial_message is correctly added, extract filebeat/input/journald/testdata/ndjson-parser.journal.gz and ingest it with the input. There is a single event on this journal file and it contains CONTAINER_PARTIAL_MESSAGE=true.

Related issues

• Closes [Journald] Better align the fields produced by the input with ECS #42208

## Use cases
## Screenshots
## Logs

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This is an automatic backport of pull request #42403 done by [Mergify](https://mergify.com).

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)

[mergify]

This pull request has not been merged yet. Could you please review and merge it @belimawr? 🙏