feat: Propagate ami_type to self-managed node group; allow using ami_type only (#3030)

modules/_user_data/outputs.tf

@@ -1,4 +1,9 @@
 output "user_data" {
   description = "Base64 encoded user data rendered for the provided inputs"
-  value       = try(local.platform[var.platform].user_data, null)
+  value       = try(local.user_data_type_to_rendered[local.user_data_type].user_data, null)
+}
+
+output "platform" {
+  description = "[DEPRECATED - Will be removed in `v21.0`] Identifies the OS platform as `bottlerocket`, `linux` (AL2), `al2023, or `windows`"
+  value       = local.user_data_type
 }