Remove "directly addressed"

What does this MR do and why?

Users either don't notice or don't understand the difference between "directly addressed" and "mentioned". Right now we only hide the "directly addressed" filter from the UI, and use the same ToDo text of "mentioned" for these items, but leave them in the system and in the API.

We will then afterwards deprecate it from the API and fully remove it in a follow-up: #361652

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