Enable security_bulk_operations_notifications FF by default

What does this MR do and why?

Enables the security_bulk_operations_notifications feature flag by default. This is a follow-up to !219509 (merged) which introduced background operation tracking and email notifications for bulk security attribute updates.

With this change, all bulk operations in Security Inventory now use the new BackgroundOperationBulkUpdateWorker with operation tracking and failure notifications enabled by default, providing users with better visibility into bulk operation status and failures.

Implementation details

• Feature flag type changed: Moved from gitlab_com_derisk to beta type (required for default_enabled: true)
• Feature flag location: Moved from config/feature_flags/gitlab_com_derisk/ to config/feature_flags/beta/
• Default behavior: All bulk operations now use the new background operation tracking system with Redis-based operation state management and email notifications on failure
• Backward compatibility: Legacy behavior is still available by disabling the feature flag

MR acceptance checklist

• Tests added or updated
• Feature flag behavior verified (enabled and disabled paths)
• Backward compatibility maintained
• Changelog entry added
• No breaking changes

Related issues and epics

• Closes #577580 (closed)
• Follow-up to !219509 (merged)

Changelog

Changelog: changed