…rrectly (#33609)

### Issue # (if applicable)

Closes #33510 

### Reason for this change

A previous change ([PR33305](#33305)) removed the `IdentityPoolRoleAttachment` L2 construct, which also changed the creation logic of the default role attachment in the `IdentityPool` L2. This not only triggered redeployments, but did not allow for redeployment at all, as the new role attachment (with a different resource hash) was trying to be created before the old one was removed. This led to failed deployments, as only one role attachment can exist per identity pool.

### Description of changes

Brought back the `IdentityPoolRoleAttachment` L2 logic to prevent redeployment for customers using CDK `<v2.179.0`. However, the construct is now no longer being exported, which preserves the original intention of preventing confusion about using this resource.

### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

`yarn test && yarn integ test/integ.identitypool.js --update-on-failed`

### 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)

**BREAKING CHANGE**: Any `IdentityPool` resources deployed in versions `>=2.179.0` will now fail to deploy. You will need to delete the `IdentityPoolRoleAttachment` from your stack via the console before redeploying.

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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*