[8.x](backport #41354) Add the missing process.name field to System module, Syslog fileset

[mergify]

Proposed commit message

This PR adds the missing process.name field to System module, Syslog fileset

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

## Author's Checklist

How to test this PR locally

0. Make sure you're testing on a system where system logs are sent to Journald. The Debian 12 Vagrant VM from Beats is a good option.
1. Package Filebeat (adjust for your system/platform). If you don't package it, dashboards won't be loaded

   DEV=true SNAPSHOT=true EXTERNAL=true PACKAGES="tar.gz" PLATFORMS=linux/amd64 mage -v package
   

2. Extract it, enable the system module

   ./filebeat modules enable system
   

3. Edit filebeat/modules.d/system.yml to enable the syslog fileset and ensure Journald input will be used

   - module: system
     syslog:
       enabled: true
       var.use_journald: true
   

4. Edit filebeat.yml with your ES and Kibana credentials

   filebeat.config.modules:
     path: ${path.config}/modules.d/*.yml
     reload.enabled: false
     reload.period: 1s
   
   setup.template:
     settings:
       index.number_of_shards: 1
   
   setup.kibana:
     host: "http://kibana:5601"
     username: admin
     password: testing
     ssl.verification_mode: none
   
   output.elasticsearch:
     hosts: ["http://elasticsearch:9200"]
     preset: latency
     protocol: "http"
   
     username: admin
     password: testing
     ssl.verification_mode: none
   

5. Run the setup command

   ./filebeat setup --modules system
   

6. Start Filebeat
7. Ensure the events contain process.name and the [Filebeat System] Syslog dashboard ECS contains data. You might have to adjust the time window

Related issues

• Fixes [Filebeat - System module - Journald] Some fields are missing #41353

## Use cases
## Screenshots
## Logs

---

This is an automatic backport of pull request #41354 done by [Mergify](https://mergify.com).

[elasticmachine]

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