Set FF `ci_remove_legacy_predefined_variables` to default ON

What does this MR do and why?

This sets the feature flag ci_remove_legacy_predefined_variables to default ON. This FF is currently being rolled out in #404533 (closed) and is expected to be fully rolled out by the end of this week.

Since we require code changes to be merged in by the 11th, I am creating this MR now to ensure it is included in the %16.0 release.

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 #404533 (closed)