Fix pipeline scheduling when the frequency is more than 1 hour

What does this MR do?

This MR fixes calculating the next_run_at of pipeline schedulers when the frequency is more than 1 hour.

Related to #333866 (closed)

In the first implementation (!62826 (merged)), we missed the usage of schedule's cron syntax when calculating the next_run candidates.

The feature flag is now disabled.

Does this MR meet the acceptance criteria?

Conformity

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• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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