lib/releaselib: Continue publishing md5/sha1 until Kubernetes 1.18
lib/releaselib: Continue publishing md5/sha1 until Kubernetes 1.18

Bumping the version guard on the md5/sha logic to assist kops.
This will break again once the 1.17 branch is cut, so any kops et al testing
logic should be reconfigured to accommodate SHA{256,512} hashes before then.

Signed-off-by: Stephen Augustus <f96984fef9637085f0559f1484dfd4e99545cd6e@vmware.com>

kubernetes/release,kubernetes/release