docs(lambda-layers): point to alternative versions (#24071)

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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: kaizencc

packages/@aws-cdk/lambda-layer-awscli/README.md

@@ -25,3 +25,8 @@ fn.addLayers(new AwsCliLayer(this, 'AwsCliLayer'));
 ```
 
 The CLI will be installed under `/opt/awscli/aws`.
+
+## Alternatives
+
+This module bundles AWS cli v1. To use AWS cli v2, you can use the
+external module [awscdk-asset-awscli](https://github.com/cdklabs/awscdk-asset-awscli/tree/awscli-v2/main).

packages/@aws-cdk/lambda-layer-kubectl/README.md

@@ -26,3 +26,10 @@ fn.addLayers(new KubectlLayer(this, 'KubectlLayer'));
 ```
 
 `kubectl` will be installed under `/opt/kubectl/kubectl`, and `helm` will be installed under `/opt/helm/helm`.
+
+## Alternatives
+
+This module bundles Kubectl v1.20.0 and the associated helm version
+To use alternative Kubectl versions, including the latest available,
+you can use the external module
+[awscdk-asset-kubectl](https://github.com/cdklabs/awscdk-asset-kubectl).