fix: DEP0044 in TerraformOutput

[gabegorelick]

Don't use deprecated util.isArray.

This was originally approved as hashicorp/terraform-cdk#3861 but never merged.

Related issue

Fixes hashicorp/terraform-cdk#3860

Description

When running on Node 22+, the following deprecation warning is logged:

[DEP0044] DeprecationWarning: The util.isArray API is deprecated. Please use Array.isArray() instead.

See https://nodejs.org/api/deprecations.html#DEP0044

There's no good reason to use util.isArray. It's an ancient API that's been superseded by Array.isArray(). It's use here seems to be totally unintentional, perhaps from bad autocomplete.

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