Default enable security_policies_kev_filter feature flag

What does this MR do and why?

Default-enables the security_policies_kev_filter feature flag by moving it from wip to beta type and setting default_enabled: true.

The feature was rolled out in production https://gitlab.slack.com/archives/C101F3796/p1775574792152219

Changelog: added

EE: true

References

Rollout issue: #576858 (closed)

Feature epic: gitlab-org#16311 (closed)

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