lib/releaselib: s/release::gcs::stage_and_hash/release::gcs::prepare_tarball
lib/releaselib: s/release::gcs::stage_and_hash/release::gcs::prepare_tarball

This function neither stages, nor hashes any artifacts.
It is responsible for creating tarballs based on a staging directory
and a set of source and destination artifacts.

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

kubernetes/release,kubernetes/release