Make `gitlab-kas` image version correctly determined automatically

Currently the version of the gitlab-kas image used in the deployment is set to latest when we use a non-tagged version of the gitlab helm chart. When using a tagged version of the chart, the tag number is correctly populated instead.

As we don't always use a tagged version of the Gitlab chart for k8s-workloads/gitlab-com we need to have a solution to allow automatic bumping of this as needed.