CBG-5268: [4.0.5 backport] Add journalctl command to collect kernel messages

[gregns1]

CBG-5268

Backports #8198

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/0000/

[Copilot]

Pull request overview

Adds an sgcollect Linux task to capture kernel messages via journalctl, complementing existing OS/system log collection in tools/tasks.py.

Changes:

• Introduces a new LinuxTask named “Kernel Messages”.
• Runs journalctl -k with short-monotonic formatting and --no-hostname output.