feat(eks): make kubectlLayer property required from optional (#32930)

### Issue
#33261

### Reason for this change

`aws-cdk-lib` has a very outdated version of kubectl layer as dependency https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/package.json#L123. It uses an outdated helm version which is involved in a CVE.

The dependency was added because if users do not provide a kubectl layer version for EKS cluster, it will use v20 as the default. CDK itself shouldn't use a specific version of kubectl layer as dependency.

To remove the dependency, `kubectlLayer` will become a required property instead of optional. The default version v20 is too old to work with current EKS supported version v24+. However, if you're not using the property, you will see an error saying it's a required property. Please uses a kubectl layer version that's compatible with your cluster.

### Description of changes

- Make the property required from options
- Update unit tests and integration tests
- Remove the dependency of `"@aws-cdk/asset-kubectl-v20": "^2.1.3"`

### Describe any new or updated permissions being added




### Description of how you validated changes

unit tests/integration tests

### Checklist
- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

BREAKING CHANGE: `kubectlLayer` property is now required in EKS `Cluster` and `FargateCluster` constructs. The default value for `kubectlLayer` is outdated and hence being removed. You can specify your own kubectlLayer version based on your Kubernetes version.

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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: xazhao

allowed-breaking-changes.txt

@@ -951,3 +951,8 @@ changed-type:aws-cdk-lib.cx_api.CloudFormationStackArtifact.notificationArns
 # See: https://github.com/cdklabs/cloud-assembly-schema/pull/121
 weakened:aws-cdk-lib.cloud_assembly_schema.MetadataEntry
 weakened:aws-cdk-lib.cx_api.MetadataEntryResult
+
+# Making kubectlLayer prop required from optional so we can remove the outdated kubectl layer dependency
+strengthened:aws-cdk-lib.aws_eks.ClusterProps
+strengthened:aws-cdk-lib.aws_eks.FargateClusterProps
+removed:aws-cdk-lib.lambda_layer_kubectl.KubectlLayer