fix(lib): Correct types for allowedAccountIds & forbiddenAccountIds in S3Backend

[Booligoosh]

Related issue

No related issue as it is only a simple 2-line change. Can create one if required.

Description

When providing these fields as a string rather than a list of strings, Terraform throws an error. This PR updates the types to be a list of strings, to allow specifying the format that Terraform requires (similar to the other S3BackendConfig keys such as sharedConfigFiles, sharedCredentialsFiles etc).

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

[hashicorp-cla-app]

All committers have signed the CLA.

[DanielMSchmidt]

Thank you 🙏

[Booligoosh]

Thanks for merging!

[github-actions]

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