Consolidate duplicate webhook logic for managed cluster types

[nojnhuh]

/kind cleanup

What needs cleanup: [A clear and concise description of what needs to be cleaned up.]

Currently, the same logic is duplicated in both the template and non-template webhooks for AzureManaged* resources:

https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/2522297040bad726d43547531da05d012368f053/api/v1beta1/azuremanagedcontrolplanetemplate_default.go#L26 https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/2522297040bad726d43547531da05d012368f053/api/v1beta1/azuremanagedcontrolplane_webhook.go#L79

Describe the solution you'd like

The webhooks for AzureManaged* resources should follow the same pattern as the self-managed cluster types to avoid duplication.

Anything else you would like to add: [Miscellaneous information that will assist in solving the issue.]

[nojnhuh]

/milestone v1.14

[mboersma]

/milestone next

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/lifecycle stale

[nawazkh]

/remove-lifecycle stale