test: add tests for terraform 1.8.x

Add tests for terraform [1.8.x](https://github.com/hashicorp/terraform/releases/tag/v1.8.0) and probably reconsider our test matrix only to test for currently supported terraform versions.

https://endoflife.date/terraform
![image](https://github.com/coder/terraform-provider-coder/assets/10648092/3c0b529e-f79a-45c9-8836-dcaf7ad67bc5)

