checksum replacement with sha256 for os_images

Daniel Antonrequested to merge
DA-checksum-sha256-replacement into main

Replace checksum element with sha256 for os_images as is not recognized by s-c-c validation schema. os-image-server MR sylva-projects/sylva-elements/helm-charts/os-image-server!32 (merged) has to be approved before this MR. Closes #985 (closed)

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