Unable to associate NSG's with Subnets

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### Versions

**terraform**: v1.8.4

**azure provider**: azurerm v3.107.0

**module**: Azure/caf-enterprise-scale/azurerm

### Description

'network_security_group_id' appears to be present to associate an existing nsg with a subnet, but it doesn't appear to work.
I've searched through the module for 'azurerm_subnet_network_security_group_association' or 'network_security_group_id' to see where this setting is used, and don't see anything.

What am I missing here?

```
configure_connectivity_resources = {
settings = {
hub_networks = [
{
enabled = true
config = {
address_space = ["10.10.0.0/20"]
location = "northeurope"
subnets = [
{
name = "snet-nteu-hub-gw"
address_prefixes = ["10.10.0.0/24"]
network_security_group_id = "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/abc-connectivity-northeurope-dev-nsg/providers/Microsoft.Network/networkSecurityGroups/nsg-snet-nteu-hub-gw"
route_table_id = ""
}
]
}
}
]
}
}
```

Start by tracing the module's handling of `network_security_group_id` and searching for `azurerm_subnet_network_security_group_association`, as the issue does. Compare that behavior with the supplied `configure_connectivity_resources` configuration and its Terraform plan. Done means establishing whether subnet NSG association is supported and what the reported configuration is missing or failing to do.

Written by the indexing model from the issue text.