how to specify in terraform code to install always the latest version of azurerm resources.
It depends on the resource.
Normally goes like this: you query the available versions using a data source, e.g. azurerm_kubernetes_service_versions
You can then reference the latest version in your resource like so:
version = data.azurerm_kubernetes_service_versions.current.latest_version
You need to be careful with this approach though. Every time a new version is available a new
terraform plan will include an update to the resource unless you specifically tell it to ignore version changes.