AWS::ElasticLoadBalancingV2::LoadBalancer.LoadBalancerAttribute routi…

…ng.http.desync_mitigation_mode (#1660)

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-elasticloadbalancingv2-loadbalancer-loadbalancerattributes.html#cfn-elasticloadbalancingv2-loadbalancer-loadbalancerattributes-key

src/cfnlint/rules/resources/elb/Elb.py

@@ -89,6 +89,7 @@ def check_loadbalancer_allowed_attributes(self, properties, path, scenario):
             ],
             'application': [
                 'idle_timeout.timeout_seconds',
+                'routing.http.desync_mitigation_mode',
                 'routing.http.drop_invalid_header_fields.enabled',
                 'routing.http2.enabled'
             ],