Deprecate the team labels

To be merged on 2019-08-19.

Relates to gitlab-org/quality/triage-ops#254 (closed).

I've set up a dashboard for ~"Platform [DEPRECATED]" to ensure that dashboards work with the [DEPRECATEd] suffix:

• https://deprecate-team-labels-review.quality-dashboard.gitlap.com/groups/gitlab-org/teams/Platform%20%5BDEPRECATED%5D
• https://deprecate-team-labels-review.quality-dashboard.gitlap.com/groups/gitlab-org/projects/gitlab-ce/teams/Platform%20%5BDEPRECATED%5D