Update Cosign in release workflow to 3.x

[vepatel]

Cosign 3.x has breaking changes, which require modification in signing script

• https://github.com/nginx/kubernetes-ingress/actions/runs/20039440674/job/57471897961#step:6:12
• https://github.com/sigstore/cosign/releases/tag/v3.0.1

[github-actions]

Hi @vepatel thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!