CKS: support headlamp.yaml in upgrade-kubernetes.sh

[mw-0]

Description

This PR fixes #14244.

Since 4.23 the binaries ISO ships the Headlamp dashboard as headlamp.yaml instead of
dashboard.yaml. The create path (k8s-control-node.yml) already falls back between the two,
but upgrade-kubernetes.sh hardcoded dashboard.yaml. Because the script runs with bash -e,
every upgrade using a Headlamp ISO failed on the first control node after kubeadm had already
upgraded it, leaving the cluster half-upgraded and in Alert.

This applies the same fallback as the create path: headlamp.yaml, then dashboard.yaml,
then a warning, so ISOs built before and after the Headlamp change both work.

Affects 4.23.0.0 and main. 4.22 is not affected. Happy to retarget if a 4.23 branch is
wanted for a patch release.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Feature/Enhancement Scale or Bug Severity

Bug Severity

• Major

How Has This Been Tested?

• 4.23.0.0 management server with the patched script, single-control/single-worker CKS
  cluster, isolated network, .
• Upgraded 1.35.8 -> 1.36.0 using a Headlamp-built ISO: upgrade completes, both nodes on
  v1.36.0, headlamp running in kube-system, cluster back to Running, version updated in DB.
• Before the patch the same upgrade failed with
  error: the path "/mnt/k8sdisk//dashboard.yaml" does not exist.

How did you try to break this feature and the system with this change?

• tried to upgrade using custom isos
• Missing both files only logs a warning instead of failing the upgrade.### Description

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[andrijapanicsb]

@ACSHomeBot package G

[ACSHomeBot]

Packaging results:

Result	Artifact	Platform
PASS	RPM	EL (EL8/9/10)
PASS	DEB	Ubuntu, Debian

Test packages are available at:

• EL9/RPM: https://f003.backblazeb2.com/file/andrijapanicsb-cloudstack-pr-builds/pr/14245/el9/
• Debian/APT: https://f003.backblazeb2.com/file/andrijapanicsb-cloudstack-pr-builds/pr/14245/debian/
• OpenPGP public key