Simplify etcd image release process

[jmhbnz]

What would you like to be added?

Currently Etcd publishes multi-arch along with arch specific images, i.e:

Now that multi-arch image manifests are mature and widely adopted should we stop creating and pushing single arch images?

We would need to consider how much lead time to give for this change and how we communicate this to folks that might be consuming the single arch image tags currently.

Why is this needed?

For the project this would simplify scripts/release.sh and scripts/test_images.sh and cut some time from the release process. For downstream users this would make the image landscape for etcd much simpler to consume.

[jmhbnz]

From the discussion in #15270 it sounds like there might be agreement to go ahead with this feature. I've created #15605 but left in draft because I think this still needs wider discussion, particularly on when it would be ok to make this change and what steps we should take to communicate it.

[stale]

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