Fix deadlink to docs for GitLab Mattermost SSO
Just fix a deadlink to docs for GitLab Mattermost SSO.
cf. The changes appeared in mattermost/platform@341d270
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Just fix a deadlink to docs for GitLab Mattermost SSO.
cf. The changes appeared in mattermost/platform@341d270