Enable child-epics-from-different-hierarchies by default

Related to #375622 (closed)

What does this MR do and why?

After deemed safe we want to enable child_epics_from_different_hierarchies feature flag by default.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.