[VULN-649] ci: Remove deprecated scan workflow

[theMickster]

🎟️ Tracking

https://bitwarden.atlassian.net/browse/VULN-649

📔 Objective

Removes the deprecated scan.yml GitHub Actions workflow.

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

This PR deletes the deprecated .github/workflows/scan.yml GitHub Actions workflow (Checkmarx SAST and check-run jobs), tracked under VULN-649. The removal is complete — no other workflow or file references the deleted scan.yml, its check-run/_checkmarx jobs, or the workflow name. As a CI-infrastructure-only change with no plugin code touched, no plugin version bump or changelog entry is required.

Code Review Details

No findings. The deletion is clean and well-scoped:

• No dangling references to the removed workflow remain in .github/ or elsewhere in the repo.
• The change is authorized by an AppSec VULN task (VULN-649), so the removal of the SAST workflow is an intentional security-team decision, not an accidental coverage gap.
• Removing the pull_request_target trigger on main eliminates a historically risky workflow pattern.