Update coreos version used in packer configuration

Overview

The mirror tag job failed for build images because it could not find the coreos image.

https://dev.gitlab.org/cookbooks/gitlab-omnibus-builder/-/pipelines/314013

Context

The version of coreos was pinned in Pin coreos version to avoid surprises (!123 - merged) • DJ Mountney • 12.3 to avoid surprises.

The universe found an alternative way to surprise us today, when the image was no longer present.

Deliverables

• Find a working version of coreos and update the version in https://gitlab.com/gitlab-org/gitlab-omnibus-builder/-/blob/92c9f9441fe08854b3c6333f7bb05aa75696736a/packer/packer-basic.json#L12
• Open issue to follow up to make this more resilient