Enable billable_members API by default

What does this MR do?

Enabling the billable_members API by default.

I feel we need to monitor and improve its performance for a few releases before the feature flag can be safely removed.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• 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.

Related to #217384 (closed)