Refactor duo cascading defaults into a single data-driven method

What does this MR do and why?

Replaces the individual reset_duo_features_to_inherit_from_namespace and reset_ai_audit_events_storage_to_inherit_from_namespace methods with a single sync_duo_cascading_defaults_from_namespace that iterates over a DUO_CASCADING_SETTINGS array. The DB default is read from ProjectSetting.column_defaults at runtime, so adding a new write-cascade setting requires only one array entry instead of a new method.

Stacked on !255460 (merged).

References

• !255460 (merged) (docs MR documenting the project-creation gap)
• !255428 (merged) (the review that prompted this cleanup)

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. Run existing specs: bundle exec rspec ee/spec/services/projects/create_service_spec.rb
2. Verify behavior is unchanged: projects created under groups with non-default duo settings inherit correctly.

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