Fix feature flag check for productivity analytics

What does this MR do?

This MR makes sure that we use the productivity_analytics_enabled? helper method to check if feature flag is enabled. The FF is enabled by default: it caused a bug, since the previous implementation didn't use the default_enabled key for the FeatureConstrainer when looking up the FF.

Feature.enabled?(:productivity_analytics) => false

Feature.enabled?(:productivity_analytics, default_enabled: true) => true

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• 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

Closes #34786 (closed)