Mark kafka_version as optional

For the free account you cannot pick kafka_version so this needs
to be optional.
For dedicated the version is instead validated server side

cloudkarafka/resource_instance.go

@@ -95,7 +95,7 @@ func (r *instanceResource) Schema(_ context.Context, _ resource.SchemaRequest, r
 			},
 			"kafka_version": schema.StringAttribute{
 				Description: "Which Apache Kafka version to use.",
-				Required:    true,
+				Optional:    true,
 				Validators: []validator.String{
 					stringvalidator.RegexMatches(
 						regexp.MustCompile(`^\d+\.\d+\.\d+$`),