Update Documentation about role permissions

What does this MR do and why?

I was running into an HTTP 403 error with a Access Token manipulating Issues and first looked at the matrix in the documentation about user rights which wasn't showing information about the minimal rights requirements for due dates of issues. Under the due date infopage, there was the information about reporter rights as miinimum. So I went ahead to create a new line in the matrix for the due dates.

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