Bump terraform-exec to v0.18.1

[austinvalle]

Closes #228

Background

A recent update of tfexec v0.18.0 caused a regression where detailed stderr information wasn't being passed to downstream modules. This has now been fixed with the new release v0.18.1

While the docs tool doesn't use this error information directly, this caused a situation where upgrading terraform-plugin-docs from 0.13.0 to 0.14.0 could cause acceptance tests to fail if the consuming module also uses terraform-plugin-testing or terraform-plugin-sdk.

Examples:

• Bump github.com/hashicorp/terraform-plugin-docs from 0.13.0 to 0.14.0 terraform-provider-tls#327
• https://github.com/hashicorp/terraform-provider-tls/actions/runs/4296411115

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.