[WIP] ⚠️ Change type of Cluster.spec.controlPlaneRef/infrastructrureRef

[sbueringer]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #6539

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[sbueringer]

/test pull-cluster-api-e2e-main

[k8s-ci-robot]

@sbueringer: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-verify-main	d198ea8	link	true	/test pull-cluster-api-verify-main
pull-cluster-api-test-main	d198ea8	link	true	/test pull-cluster-api-test-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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