Remove the old deprecated state, and rename discouraged to deprecated

The old Discouraged state was typically what most companies call Deprecated, and in fact how it is generally used within GitLab. We should remove the old definition of deprecated, and rename discouraged to deprecated to align with general practices.

MySQL is also referenced as discouraged in /ce/install/requirements.html#database. A separate MR is opened to update that language which should be merged on this change has been approved: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24544