Drop ci_atomic_processing_ordered_update_stage feature flag

What does this MR do and why?

!147875 (merged) introduced the ci_atomic_processing_ordered_update_stage feature flag to roll out a fix for a bug that occurred when jobs within the same stage used needs in a directed acyclic graph (DAG).

This has been enabled on production with no issues, so let's make this the default.

Relates to #451992 (closed)

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