feat: add support for terraform v1.9 (#949)

Run tests for terraform 1.9.x that was released last week:
https://github.com/hashicorp/terraform/releases/tag/v1.9.0

.github/workflows/test.yml

@@ -48,6 +48,8 @@ jobs:
             version: v1.7.x
           - tool: terraform
             version: v1.8.x
+          - tool: terraform
+            version: v1.9.x
 
     steps:
       - uses: actions/checkout@v4

README.md

@@ -8,7 +8,7 @@
 ## Requirements
 
 - [Terraform](https://developer.hashicorp.com/terraform/downloads)
-  - HashiCorp recommends to use the two latest terraform releases (1.8.x). Our test suite validates that our provider works with these versions.
+  - HashiCorp recommends to use the two latest terraform releases (1.8.x, 1.9.x). Our test suite validates that our provider works with these versions.
   - This provider uses the [terraform plugin protocol version 6](https://developer.hashicorp.com/terraform/plugin/terraform-plugin-protocol#protocol-version-6), and should work with all tools (ie. Terraform & OpenTofu) that supports it.
 - [Go](https://go.dev/doc/install) 1.21.x (to build the provider plugin)