Define Work Items terminology

What does this MR do and why?

See the Plan team agenda item here (internal).

To avoid confusion that has arisen while designing and building Work Items, we need to enforce an agreed terminology that clearly describes new and legacy components.

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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.