Switch to gitlab_rails['auto_migrate'] = false

Closed username-removed-5332 requested to merge bbodenmiller/gitlab-ce:patch-11 into master

What does this MR do?

Update docs to use gitlab_rails['auto_migrate'] = false rather than /etc/gitlab/skip-auto-migrations per https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1685#note_17823027.