Remove security_policies_kev_filter feature flag (backend)

What does this MR do and why?

This change removes a feature flag called security_policies_kev_filter that was controlling whether CVE (security vulnerability) enrichment information would be displayed in security policy violations.

Previously, this enrichment data (which shows additional details like exploit probability scores and whether a vulnerability is known to be actively exploited) was only shown when the feature flag was enabled. Now, this enrichment information will always be displayed by default.

The change also removes all the conditional logic and tests that were checking whether this feature flag was enabled or disabled, simplifying the codebase. Additionally, it includes a minor code style improvement by removing unnecessary parentheses in a string concatenation.

Essentially, this makes a security feature that was previously optional now permanently available to all users, providing them with richer information about security vulnerabilities without needing to toggle any settings.

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Related to #576859 (closed)