Enable Vulnerability Management Policy Severity Override

What does this MR do and why?

This MR enables the security_policies_severity_customize feature flag by default, making the Vulnerability Management Policy Severity Override feature generally available for all users.

Changes:

• Changes the feature flag type from wip to development
• Sets default_enabled to true
• Renames the feature flag file from ee/config/feature_flags/wip/ to ee/config/feature_flags/development/

This feature allows security policies to override vulnerability severities, enabling organizations to customize severity levels based on their own risk assessment criteria.

Changelog: added EE: true

References

• Rollout issue: #588540 (closed)
• Feature flag introduced in: !222151 (merged)
• Parent epic: Automated Vulnerability Severity Overrides/Customization Policy

MR acceptance checklist

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Related to #588540 (closed)