Update mattermost.rb
Updating to Mattermost v3.6.1, see https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1868 for more details
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Updating to Mattermost v3.6.1, see https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1868 for more details