Do not show 13.13-pre on the header

After 13.12 was released on 2021-05-22, we are seeing "13.13-pre" on the header.

It should be "14.0-pre".

https://gitlab.com/gitlab-org/gitlab-docs/-/blob/5960afb83cfb29e73bc7bfa34acb5f05fb90d275/lib/helpers/versions.rb#L64-69

cf. #689 (closed)