…ack name for Application stack (#24171)

* Assign value of `stackName` to `stackId` for Application stack, so the stack id will always be the same as stack name. Users wanting to control stack id can do so via `stackName`.

Closes #24160.

Background:
* Customers wish to control or modify the stack id of the Application stack to follow their project conventions within CDK.
* In previous [fix](#23823), we had deprecated the stack id to push users towards using stack name as the main mechanism for stack identification.

Note on backward-compatibility:

After this change, the `stackId` parameter can no longer be used to control the application stack id. The stack id will always match stack name, and the default stack name if not specified will be `Application-${APPLICATION_IDENTIFIER}-Stack`. `${APPLICATION_IDENTIFIER}` is application name for CreateTargetApplication and application id for ExistingTargetApplication.

If you created an application stack prior to aws-cdk [release v2.64.0](https://github.com/aws/aws-cdk/releases/tag/v2.64.0) and did not specify a stack id or name, you may run into the following error during deployment due to the creation attempt of a new stack with the same application:
```log
Resource handler returned message: "You already own an application 'MyApplicationName' (Service: ServiceCatalogAppRegistry, Status Code: 409, Request ID: xxxx)" (RequestToken: yyyy, HandlerErrorCode: InvalidRequest)
```

To address this error, you can set the `stackName` value to match your existing stack. For example:
```typescript
const associatedApp = new ApplicationAssociator(app, 'MyApplicationAssociator', {
  applications: [ TargetApplication.createApplicationStack({
    applicationName: 'MyApplicationName',
    stackName: 'ApplicationAssociatorStack', // Add your existing stack name here
    ...
```
This will result in the existing stack deploying with the previous name, and the id in CDK will reflect this same stack name.

Related links:
* Previous PR: #23823
* Previous GitHub issue: #23861
* Original reason that introduced `stackId`: #22508

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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*