Upgrade jenkins image as preparation for migration to upstream

With those modifications, the dockerfile inherits from the tag 4.11 instead of latest (at the time of writing latest pulls from 4.13). This tag has the exact same version that users will have installing the upstream template, so installing another version of jenkins is no more needed. Gitlab CI jobs are set all manual but image building, because they are not updated to work on okd4 and not worth upgrading.