[release-2.10] 🐛 fix: Prevent volumeSize updates in ROSAMachinePool

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #6086

/assign serngawy

Fix volumeSize immutability for ROSAMachinePool to prevent OCM API errors when updating other fields. volumeSize is now enforced as immutable via CEL validation and is no longer sent in UPDATE requests.

[kubernetes-prow]

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[kubernetes-prow]

@k8s-infra-cherrypick-robot: The following test 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-provider-aws-test-release-2-10	68415e7	link	true	/test pull-cluster-api-provider-aws-test-release-2-10

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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[damdo]

/hold