chore(ci): improve generate kubeconfig for nested cluster

[universal-itengineer]

Description

Update gen-kubeconfig.sh to search for the kubernetes-api ingress in kube-system as a fallback after d8-user-authn.

Why do we need it, and what problem does it solve?

In Deckhouse 1.76, the kubernetes-api ingress moved to the kube-system namespace. Without the fallback, kubeconfig generation for nested clusters fails on newer Deckhouse versions.

What is the expected result?

gen-kubeconfig.sh successfully generates kubeconfig on clusters where kubernetes-api ingress is located either in d8-user-authn or in kube-system.

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: ci
type: chore
summary: Support Kubernetes API ingress lookup in kube-system during nested cluster kubeconfig generation.
impact_level: low