fix(eks): incorrect nodegroupName(under feature flag) (#29794)

### Issue # (if applicable)

As #22910 and #25278 were closed for staleness. I am opening a new PR to fix #22442.

To avoid breaking change, this PR introduces a new feature flag for the bug fix.

Closes #22442

### The Code for verification

```ts
import {
  App, Stack,
  aws_eks as eks,
  aws_ec2 as ec2,
  CfnOutput, cx_api,
} from 'aws-cdk-lib';

const app = new App();
const env = { region: process.env.CDK_DEFAULT_REGION, account: process.env.CDK_DEFAULT_ACCOUNT };
const stack = new Stack(app, 'my-test-stack2', { env });

stack.node.setContext(cx_api.EKS_NODEGROUP_NAME, true);

const vpc = ec2.Vpc.fromLookup(stack, 'Vpc', { isDefault: true });

const cluster = new eks.Cluster(stack, 'Cluster', {
  vpc,
  version: eks.KubernetesVersion.V1_29,
  defaultCapacity: 0,
});

const ng1 = new eks.Nodegroup(stack, 'ng-1', {
  cluster,
});

const ng2 = cluster.addNodegroupCapacity('ng-2', {
  nodegroupName: 'custom-name',
});

new CfnOutput(stack, 'ng1-name', { value: ng1.nodegroupName });
new CfnOutput(stack, 'ng2-name', { value: ng2.nodegroupName });
```

The output is like

```
Outputs:
my-test-stack2.ng1name = ng19F1EA947-sSziNMzPVkik
my-test-stack2.ng2name = custom-name
```

Console:

<img width="315" alt="image" src="https://github.com/aws/aws-cdk/assets/278432/019255bd-7d05-43ad-8624-baf5b6eea725">



### Checklist
- [x] 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)

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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: pahud