contrib: Make version parsing more lenient in digest pull
contrib: Make version parsing more lenient in digest pull

Make the version handling more flexible by taking in a version number in
either the "vX.Y.Z" or "X.Y.Z" formats, stripping the leading v and
setting the URL up correctly to pull the manifests from the target URL.

Fixes the following error:

    $ contrib/release/pull-docker-manifests.sh joestringer 1.7.15
    https://github.com/cilium/cilium/actions/runs/637686978
    curl: (3) URL using bad/illegal format or missing URL

    Signal ERR caught!

    Traceback (line function script):
    71 main contrib/release/pull-docker-manifests.sh
    85 main contrib/release/pull-docker-manifests.sh

    Exiting...

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium