Default enable cascading settings feature flag

What does this MR do?

Related to feature flag rollout issue #327230 (closed)

This change default-enables the feature and updates the rollout issue reference. The feature is now enabled on GitLab.com. The flag is still present so we can disable at any time if problems arise.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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• Documentation (if required)
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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