Use last_edited_at instead of updated_at for displaying issuable last change

This is EE version of gitlab-ce!16406. Although there should not be any EE conclicts in CE version, in this case it's better to use EE explicit MR to assure that no other tests are broken (issuable code is used also by epics which is only in EE).

This MR is related also to EE only issue #4416 (closed)