Configurable parallel batched migrations limit

What does this MR do and why?

Instead of hard-coded value, use application setting to control the maximum number of batched migrations executed in parallel.

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

Related to #372316 (closed)