Add changelog entry for post-processing change

[mbg]

Adds a changelog entry for #3222

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

N/A

How did/will you validate this change?

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If something goes wrong after this change is released, what are the mitigation and rollback strategies?

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How will you know if something goes wrong after this change is released?

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Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR adds a changelog entry documenting a behavioral change in the CodeQL Action's post-processing of SARIF files. The change ensures that post-processing steps are always performed regardless of upload settings, which may affect Advanced Setup users who use non-default upload input values.

• Adds a changelog entry for PR #3222 that modifies SARIF post-processing behavior