🐛 Change KCP upgrade to use machinelist for scaling decision

[jan-est]

status.Nodes is not the resource status. Method which is populating this field is using the cached client,
which is not ideal when taking decision about scale up or down. This PR change KCP to use list of machines in the controlPlane object for making scaling decision.

Fix issue commented here #4293 (comment)

[k8s-ci-robot]

Hi @jan-est. Thanks for your PR.

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[fabriziopandini]

/ok-to-test

[fabriziopandini]

/lgtm

[fabriziopandini]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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