fix(deps): update module github.com/hashicorp/terraform-exec to v0.21.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
github.com/hashicorp/terraform-exec	v0.20.0 -> v0.21.0

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Release Notes

hashicorp/terraform-exec (github.com/hashicorp/terraform-exec)

### [`v0.21.0`](https://togithub.com/hashicorp/terraform-exec/releases/tag/v0.21.0) [Compare Source](https://togithub.com/hashicorp/terraform-exec/compare/v0.20.0...v0.21.0) ENHANCEMENTS: - tfexec: Add `-allow-deferral` to `(Terraform).Apply()` and `(Terraform).Plan()` methods ([#​447](https://togithub.com/hashicorp/terraform-exec/pull/447))

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