Comment 8 for bug 1843497

Fix PR: https://github.com/charmed-kubernetes/layer-etcd/pull/160
Doc PR: https://github.com/charmed-kubernetes/kubernetes-docs/pull/281

The fix makes it possible to upgrade the etcd charm after the 2.3->3.x migration without losing data, and it prevents newly elected leader units from clobbering the existing cluster-token.

Given that upgrading to etcd 449 will still be dangerous, and users may attempt to do so as part of upgrading through CK 1.15, we will add a special upgrade note for this case.