Update `bump_etcd_version_k8s.md` to mark the step `Build etcd image` deprecated

[ahrtr]

What would you like to be added?

We use etcd officially released image, and we don't build etcd image using the legacy cluster/images/etcd anymore.

So we need to update bump_etcd_version_k8s.md.

• Let's just mark it deprecated for now
• Remove it when we remove cluster/images/etcd from Kubernetes repo

cc @hakman @ivanvc @joshjms

Why is this needed?

.

[ahrtr]

cc @yashsingh74