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:

cluster-api-provider-azure/api/v1beta1/azuremanagedcontrolplanetemplate_default.go

Line 26 in 2522297

setDefault[*string](&mcp.Spec.Template.Spec.NetworkPlugin, ptr.To(AzureNetworkPluginName))

cluster-api-provider-azure/api/v1beta1/azuremanagedcontrolplane_webhook.go

Line 79 in 2522297

setDefault[*string](&m.Spec.NetworkPlugin, ptr.To(AzureNetworkPluginName))

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