fix(elbv2): BaseLoadBalancer.vpc is not optional (#18474)

The `BaseLoadBalancer.vpc` property fulfills the `IApplicationLoadBalancer.vpc` and
`INetworkLoadBalancer.vpc` properties, both of which are optional. A bug in `jsii` (see aws/jsii#3342)
makes this pass type system consistency checks, when it should not be allowed.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: Romain Marcadier

Diff for: allowed-breaking-changes.txt

@@ -115,4 +115,9 @@ removed:@aws-cdk/aws-lambda-event-sources.SelfManagedKafkaEventSourceProps.retry
 removed:@aws-cdk/aws-lambda-event-sources.SelfManagedKafkaEventSourceProps.tumblingWindow
 base-types:@aws-cdk/aws-lambda-event-sources.KafkaEventSourceProps
 base-types:@aws-cdk/aws-lambda-event-sources.ManagedKafkaEventSourceProps
-base-types:@aws-cdk/aws-lambda-event-sources.SelfManagedKafkaEventSourceProps
+base-types:@aws-cdk/aws-lambda-event-sources.SelfManagedKafkaEventSourceProps
+
+# fixed vpc property of BaseLoadBalancer so it correctly implements I(Application|Network)LoadBalancer (i.e: must be optional)
+changed-type:@aws-cdk/aws-elasticloadbalancingv2.ApplicationLoadBalancer.vpc
+changed-type:@aws-cdk/aws-elasticloadbalancingv2.BaseLoadBalancer.vpc
+changed-type:@aws-cdk/aws-elasticloadbalancingv2.NetworkLoadBalancer.vpc

Diff for: packages/@aws-cdk/aws-elasticloadbalancingv2/lib/shared/base-load-balancer.ts

@@ -192,8 +192,11 @@ export abstract class BaseLoadBalancer extends Resource {
 
   /**
    * The VPC this load balancer has been created in.
+   *
+   * This property is always defined (not `null` or `undefined`) for sub-classes of `BaseLoadBalancer`.
    */
-  public readonly vpc: ec2.IVpc;
+  public readonly vpc?: ec2.IVpc;
+
   /**
    * Attributes set on this load balancer
    */