Latest /flexibleServers azure resource manager template references are not available in the docs.

[jannef]

Latest version available in the docs is 2023-03-01-preview.
https://learn.microsoft.com/en-us/azure/templates/microsoft.dbforpostgresql/flexibleservers?pivots=deployment-language-bicep

Deployments with later version however success without any issues:

resource postgre 'Microsoft.DBforPostgreSQL/flexibleServers@2023-06-01-preview' = {
  name: 'postgre-${name}-${environment}-${region}-${instance}'
  location: location
...
}

Some options available in the later versions are not documented, such as properties.network.publicNetworkAccess