Add feature flag to control version of Fix Pipeline flow

What does this MR do and why?

Add feature flag to control version of Fix Pipeline flow

The feature flag controls whether fix_pipeline_next is used. The "Next" version contains substantial changes to the UX and cost optimizations.

In feat: Update current MR when pipeline fails (gitlab-org/modelops/applied-ml/code-suggestions/ai-assist!4756 - merged) we are introducing fix_pipeline_next and that MR needs to be merged before we can enable the feature flag.

Related to Allow Fix Pipeline flow to iterate on the same MR (#589268)

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