docs: update default trace level from 'warning' to 'warn' in CLI documentation

[Gijsreyn]

PR Summary

Fixes #405

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[Copilot]

Pull request overview

Note

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Updates the DSC 3.0 CLI reference documentation to reflect the warn trace/log level naming (instead of warning) and its use as the default.

Changes:

• Renamed the documented message level from warning to warn (default).
• Updated the documented default value to warn.
• Updated the documented valid values list to include warn instead of warning.

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[learn-build-service-prod]

PoliCheck Scan Report

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[learn-build-service-prod]

Learn Build status updates of commit 9409e14:

✅ Validation status: passed

File	Status	Preview URL	Details
dsc/docs-conceptual/dsc-3.0/reference/cli/index.md	✅Succeeded	View (dsc-3.0)

For more details, please refer to the build report.