k8s-version-upgrade: ignore CoreDNS preflight on kubeadm upgrade plan too

The prior commit added the CoreDNS ignore/skip flags only to `kubeadm upgrade
apply`, but `kubeadm upgrade plan` runs the SAME CoreDNS preflight. Once master's
kubeadm binary is on the target version (the first attempt's apt step already
bumps it), both plan calls fail on the Keel-drifted CoreDNS 1.12.4 under
set -euo pipefail and abort:
  - preflight Job step 4 (upgrade-step.sh) — `plan` output is grepped for the
    target version; the failing pipeline killed the whole preflight.
  - update_k8s.sh master path line 85 — bare `plan` before the apply.

Both now pass --ignore-preflight-errors=CoreDNSMigration,CoreDNSUnsupportedPlugins.
Verified read-only on master: plan exits 0 and still emits
"kubeadm upgrade apply v1.34.9".

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/update_k8s.sh

@@ -82,7 +82,10 @@ sudo apt-get install -y "kubeadm=$RELEASE-*"
 
 if [[ "$ROLE" == "master" ]]; then
     echo "==> Master path: kubeadm upgrade plan + apply"
-    sudo kubeadm upgrade plan
+    # `plan` runs the same CoreDNS preflight as `apply`, so once master's kubeadm
+    # is on the new version it fails here too (under set -e) — ignore the same
+    # two CoreDNS checks. See the apply block below for the full rationale.
+    sudo kubeadm upgrade plan --ignore-preflight-errors=CoreDNSMigration,CoreDNSUnsupportedPlugins
     # The first apply may fail with "static Pod hash for component <X> did
     # not change after 5m0s" — kubeadm's 5min wait for the kubelet to reload
     # a static pod is too tight on our cluster (apiserver-to-kubelet status