Convert "issue" todo type to "work_item"

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From #513937 (comment 2419773278):

I picked this up today and opened !186077 (merged) to remove the "Work item" filter option. That was the easy part.

But I just discovered a bug while checking what else needs to be done:

At some point in the past, filtering by "Epic" broke! AFAIU the bug, when you mention someone on an Epic, that todo's target_type is Issue 😞

See this comment: &13201 (comment 2419731067) I just pinged myself on an old epic there, but in my todo dashboad it shows as this:

This used to work. I have plently of pings on epics in my Done tab, and these show up with the Epic category filter.

With work items, every work item is added to the issues table, so I believe the target_type will be "Issue" for all of them, epics included. We'll probably need to change the target_type to use work_item_type from the issues table.

It has been pretty recent since we moved epics to work items (and therefore the issues table) so that's probably when filtering by category stopped working.