Add DAST .latest template

Proposal

GitLab's DAST has always had only a single template that isn't versioned and is updated whenever we need to change things. This has proven to be difficult for users whose tests break after an update of the template. In order to give users a warning that changes are coming and to enable them to test the changes for themselves before being forced to switch, we will introduce an .latest version of the template. This template will include all new template changes made between major releases, including breaking changes.

Breaking changes will be brought into the stable template on the next major GitLab version. For non-breaking changes, these could be brought into the stable template after living in the latest template for a few releases.