fix(aws-events-targets): Consume IRestApi as target (aws#16423)

Fixes: aws#16423

When creating an Event Bridge target for an API Gateway, the
superinterface IRestApi should be consumed instead of the
concrete class RestApi.

----

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

packages/@aws-cdk/aws-events-targets/lib/api-gateway.ts

@@ -74,7 +74,7 @@ export interface ApiGatewayProps extends TargetBaseProps {
  */
 export class ApiGateway implements events.IRuleTarget {
 
-  constructor(public readonly restApi: api.RestApi, private readonly props?: ApiGatewayProps) {
+  constructor(public readonly restApi: api.IRestApi, private readonly props?: ApiGatewayProps) {
   }
 
   /**