serve k8s 1.30/1.31/1.32 OS images only for current Sylva version

sylva-projects/sylva-core!6005 (merged) was merged in sylva-core main, with an effect that I didn't have in mind: the file in this repo which configures os_image_server_additional_selectors to let the management cluster serve different versions of Kubernetes, because they do not select a particular oci-image-registry-key, will now result in having os-image-server download and serve 3 images for each Kubernetes version; eg. serve-k8s-1.32-image.yml will result in os-image-server serving the 1.32 image of sylva, the 1.32 image of sylva-n-1, and the 1.32 image of sylva-n-2.

This results in slower reconciliation of os-image-server and more likely CI breakage because higher the risk become to hit docker.io rate limiting as in https://gitlab.com/sylva-projects/sylva-core/-/jobs/12125463892