Upgrade to OpenTofu 1.8.0 and expect to apply the improvements

[yunkon-kim]

Ref.: https://opentofu.org/blog/help-us-test-opentofu-1-8-0-alpha1/

• from .tf to .tofu
• from var to backend

Override files for OpenTofu: keeping compatibility

Since we are now adding features to OpenTofu that are not present in Terraform, we want to give module authors the ability to write code for both OpenTofu and Terraform without needing to maintain two copies of their modules. You can now create files named .tofu that are exclusive to OpenTofu. If you create a file named foo.tofu, OpenTofu will ignore the similarly-named foo.tf file. You can use this functionality to put your Terraform-specific code in the .tf file and then override it for OpenTofu in the .tofu file.

[yunkon-kim]

Upgraded version to 1.8.3 by #74