fix(lib): Correct types for allowedAccountIds & forbiddenAccountIds i…

…n S3Backend (#3669)

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### 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

- [x] I have updated the PR title to match [CDKTF's style
guide](https://github.com/hashicorp/terraform-cdk/blob/main/CONTRIBUTING.md#pull-requests-1)
- [x] I have run the linter on my code locally
- [x] 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~
- [x] My changes generate no new warnings
- [ ] ~I have added tests that prove my fix is effective or that my
feature works if applicable~
- [x] New and existing unit tests pass locally with my changes

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packages/cdktf/lib/backends/s3-backend.ts

@@ -69,7 +69,7 @@ export interface S3BackendConfig {
    * (Optional) List of allowed AWS account IDs to prevent potential destruction of a live environment.
    * Conflicts with forbidden_account_ids.
    */
-  readonly allowedAccountIds?: string;
+  readonly allowedAccountIds?: string[];
   /**
    * (Optional) File containing custom root and intermediate certificates.
    * Can also be set using the AWS_CA_BUNDLE environment variable.
@@ -91,7 +91,7 @@ export interface S3BackendConfig {
    * (Optional) List of forbidden AWS account IDs to prevent potential destruction of a live environment.
    * Conflicts with allowed_account_ids.
    */
-  readonly forbiddenAccountIds?: string;
+  readonly forbiddenAccountIds?: string[];
   /**
    * (Optional) URL of a proxy to use for HTTP requests when accessing the AWS API.
    * Can also be set using the HTTP_PROXY or http_proxy environment variables.