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>

stacks/k8s-version-upgrade/scripts/upgrade-step.sh

@@ -325,9 +325,13 @@ phase_preflight() {
     exit 1
   fi
 
-  # 4. kubeadm upgrade plan matches target
+  # 4. kubeadm upgrade plan matches target. `plan` runs the same CoreDNS
+  # preflight as `apply`; once master's kubeadm is on the new version it errors
+  # on a Keel-drifted CoreDNS (start version unsupported) and, under pipefail,
+  # aborts this whole check. Ignore the two CoreDNS checks here too so plan
+  # still emits its "kubeadm upgrade apply vX.Y.Z" line. (See update_k8s.sh.)
   local plan_target
-  plan_target=$(ssh "${SSH_OPTS[@]}" "wizard@k8s-master$NODE_DOMAIN" 'sudo kubeadm upgrade plan' \
+  plan_target=$(ssh "${SSH_OPTS[@]}" "wizard@k8s-master$NODE_DOMAIN" 'sudo kubeadm upgrade plan --ignore-preflight-errors=CoreDNSMigration,CoreDNSUnsupportedPlugins' \
     | grep -oE 'kubeadm upgrade apply v[0-9]+\.[0-9]+\.[0-9]+' \
     | grep -oE 'v[0-9]+\.[0-9]+\.[0-9]+' | head -1 | tr -d v)
   if [ "$plan_target" != "$TARGET_VERSION" ]; then