Decomposed db:rollback pipeline

What does this MR do and why?

Add a version of the db:rollback job that runs with a decomposed pipeline, as part of #345653 (closed)

How to set up and validate locally

This only runs in CI. I've prepared two example MRs, one with a decomposed pipeline and one without, to illustrate the behavior here:

• Decomposed: !76703
• Not decomposed: !76704

You can see in these MRs that the new job only runs if the ~"pipeline:run-decomposed" label is set.

MR acceptance checklist

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