Replaces the hardcoded `gcr.io/etcd-development/etcd:v3.5.9` in `_clean_etcd_keeping_certs` with a dynamic ref captured from the running Kind node via `crictl`, persisted to `{backup_dir}/etcd-image.txt` and reused on subsequent cleanup runs. Self-adapts to Kind upgrades, no version table to maintain.
Testing on Kind v0.32 / etcd 3.6 surfaced two additional bugs in the whitelist cleanup that this PR does **not** fix (see so-o2o comments):
(a) the restore step pipes raw protobuf values through bash `echo`, corrupting binary bytes;
(b) the whitelist omits cluster-admin RBAC, SAs, and bootstrap tokens needed by kubeadm's pre-addon health check.
Merging this narrow fix + diagnosis trail; follow-up branch will replace the etcd-surgery approach with a kubectl-level Caddy secret backup/restore.
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