Change issues and MRs sorting type “Last updated” to “Updated date”

https://gitlab.com/gitlab-org/gitlab-ce/issues/39849 implements a sorting direction control (ascending/descending) for the issues and merge request lists. However, as mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/39849#note_94256483:

We should probably change the wording of Last updated to just Updated date, in line with Created date and Due date

Updated date is independent from sorting direction, where as Last updated implies a sorting direction.