Enable new_security_dashboard_pdf_export flag by default

What does this MR do and why?

Enable new_security_dashboard_pdf_export flag by default

The PDF export button on the new project and group security dashboards is ready for general availability. Promote the feature flag from wip to ops and enable it by default so all users can export the dashboard as a PDF without needing an administrator to enable it manually.

Also documents the feature in the security dashboard documentation, following the established history block convention.

Changelog: added
EE: true

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