🐛 Fix control plane upgrade test etcd and dns image upgrade

[CecileRobertMichon]

What this PR does / why we need it: The control plane upgrade helper in the test framework was overriding Etcd/DNS config by setting the whole struct instead of just updating the image tag. This was causing validation errors when values were previously set and getting removed as these fields are immutable, eg:

        admission webhook "validation.kubeadmcontrolplane.controlplane.cluster.x-k8s.io" denied the request: KubeadmControlPlane.controlplane.cluster.x-k8s.io "k8s-upgrade-and-conformance-orktd6-control-plane" is invalid: spec.kubeadmConfigSpec.clusterConfiguration.etcd.local.dataDir: Forbidden: cannot be modified�[0m

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[fabriziopandini]

@CecileRobertMichon thanks for fixing this!
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fabriziopandini

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[CecileRobertMichon]

/cherry-pick release-1.0

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: new pull request created: #5896

In response to this:

/cherry-pick release-1.0

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[CecileRobertMichon]

/cherry-pick release-0.4

[k8s-infra-cherrypick-robot]

@CecileRobertMichon: new pull request created: #6127

In response to this:

/cherry-pick release-0.4

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