Prefer vulnerability author for triggering resolution workflows

What does this MR do and why?

Currently we are preferring project's first human owner for triggering workflows and using vulnerability author as a fallback but now we are reversing that by preferring author and using human owner as fallback.

This change is behind a feature flag prefer_vulnerability_author_for_workflows.

https://gitlab.com/gitlab-org/gitlab/-/work_items/596910

How to set up and validate locally

1. Enable feature flag Feature.enable(:prefer_vulnerability_author_for_workflows).
2. Trigger workers for workflows for SAST FP, SAST VR and Secret FP in rails console

Vulnerabilities::TriggerFalsePositiveDetectionWorkflowWorker.perform_async(<vulnerability_id>)
Vulnerabilities::TriggerResolutionWorkflowWorker.perform_async(<vulnerability_flag_id>)
Vulnerabilities::TriggerSecretDetectionFalsePositiveDetectionWorkflowWorker.perform_async(<vulnerability_id>)

3. All of these should get triggered without error

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