Rename last_run_date to next_run_date

What does this MR do and why?

This is a follow-up MR to !90620 (merged).

• This MR changes the column name last_run_date of iterations_cadences table to next_to_run_date.

  Why? last_run_date is a confusing name. The column stores dates on which a service must run in the future.

• An unused index for cadence_create_iterations_automation is dropped.

  The index got added for the scope for_automated_iterations (!56946 (diffs).) The scope' s been removed For historical context, see !62348 (merged) (the scope&index got added) and !90620 (merged) (the scope got removed.)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #365743 (closed)