Postgresql Flexible Servers publicNetworkAccess is readonly, despite being clickable in the portal

I'm trying to set `publicNetworkAccess` for a flexible server, but the rest API has it marked as readonly.

For context, I'm trying to do this in Pulumi, which uses these specs as a source.

[Linked issue in the Pulumi repo](https://github.com/pulumi/pulumi-azure-native/issues/2860)

[JSON spec](https://github.com/Azure/azure-rest-api-specs/blob/14d24d17491d8c2bde24532cb8cc2d663c0ffd9f/specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/stable/2022-12-01/FlexibleServers.json#L905) for the property

[Microsoft docs](https://learn.microsoft.com/en-us/rest/api/postgresql/flexibleserver/servers/create?view=rest-postgresql-flexibleserver-2022-12-01&tabs=HTTP#network) for the object

For complete, this is the checkbox in Azure Portal :D:

Start with the publicNetworkAccess definition at the referenced FlexibleServers.json line and compare it with the Microsoft REST API documentation and the Azure Portal behavior described in the issue. Confirm the intended mutability and ensure the specification accurately represents it, then check the linked Pulumi issue for downstream expectations.

Written by the indexing model from the issue text.