fix(core): add RetainExceptOnCreate to ParseDeletionPolicy method

[dineshkatamneni]

Describe the reason for this change, what the solution is, and any

CFN recently added support for a new deletion policy called RetainExceptOnCreate. This policy was added to the removal-policy enum in e0d5ca, however it is missing from the switch/case block in parseDeletionPolicy, which causes errors when this removal policy is used.

Closes #26863.

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[aws-cdk-automation]

The pull request linter has failed. See the aws-cdk-automation comment below for failure reasons. If you believe this pull request should receive an exemption, please comment and provide a justification.

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[dineshkatamneni]

Exemption Request: it's a small change, and the function doesn't have any test cases before.

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

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[mergify]

Thank you for contributing! Your pull request will be updated from main and then merged automatically (do not update manually, and be sure to allow changes to be pushed to your fork).