Turn FF `ci_fix_max_includes` to default ON

What does this MR do and why?

This MR turns the FF ci_fix_max_includes to default ON. It was introduced in Fix to prevent unlimited number of CI includes (!112963 - merged) and successfully rolled out globally in #390909 (closed).

The change behind this FF fixes a bug that formerly allowed unlimited nested includes in a CI config. We decided to turn this FF to default ON prior to removal in order to ensure self-managed instances have a sufficient adjusting period.

MR acceptance checklist

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Related to #390909 (closed)