Default enable project_members_index_by_project_namespace

What does this MR do and why?

project_members_index_by_project_namespace has been globally enabled and is ready to be default enabled.

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.

Closes #390464 (closed)