don't set LATEST_TAG automatically in patch update guide
As discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/2607, this removes the automatic setting of the LATEST_TAG variable in the update guide, because it could get set to rc versions.
Instead, a command was added to list all tags, with the most recent one being on the end of the list. Also a note was added to prevent users from updating to new minor/major versions because the tag list includes all tags.
[ci skip]
See merge request !1510
