Enable paginated_mr_discussions feature by default

What does this MR do and why?

Enables paginated_mr_discussions feature flag by default so self-managed instances will benefit from it as well.

Leaving it to this state for at least a single milestone so it can be disabled if an issue occurs.

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.

Related to #364497 (closed)