chore(ecs-service-extensions): update readme to reference v2 migration guide (#19907)

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/master/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: kaizencc

packages/@aws-cdk-containers/ecs-service-extensions/README.md

@@ -9,6 +9,11 @@
 
 <!--END STABILITY BANNER-->
 
+> ⚠️ v2 of this library is now available! It is compatible with AWS CDK v2 and available in
+> multiple languages.
+> The source code for v2 lives [here](https://github.com/cdklabs/cdk-ecs-service-extensions).
+> To migrate from this library to v2, see the [Migration Guide](https://github.com/cdklabs/cdk-ecs-service-extensions/blob/main/MIGRATING.md).
+
 This library provides a high level, extensible pattern for constructing services
 deployed using Amazon ECS.