bump k8s version in AKS due to 1.26 reaching EOL

[orfeas-k]

Context

AKS moves fast with Kubernetes versions released https://learn.microsoft.com/en-us/azure/aks/supported-kubernetes-versions?tabs=azure-cli#aks-kubernetes-release-calendar. Thus, we need to follow in order to have the CI running. Currently, the CI is failing since 1.26 is not supported any more.

What needs to get done

Bump k8s version from 1.26 to 1.28 since it's the latest supported version in for CKF. Note that we could also do 1.29 since in #756 we evaluated that 1.29 is compatible with k8s.

Definition of Done

AKS workflows use 1.28 version

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/KF-5648.

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[orfeas-k]

Katib UAT fails but it failed previously as well #893
Training-operator UAT starts to failing after bumping k8s version #894

We need to investigate more