docs: fix misleading GCS service account impersonation claims

- Remove impersonate_service_account from example configurations
- Update authentication methods to remove impersonation reference
- Add clear limitation notice that impersonation is not yet implemented
- Remove commented impersonation example in configuration

This ensures users understand current limitations and prevents
confusion about unsupported features.

Author: shirkevich

website/docs/core-concepts/stacks/yaml-functions/terraform.state.mdx

@@ -368,8 +368,6 @@ components:
           bucket: "my-terraform-state"
           prefix: "terraform/state"
           credentials: "/path/to/service-account-key.json"
-          # Or use impersonation:
-          # impersonate_service_account: "[email protected]"
       vars:
         # Read state from another component
         vpc_id: !terraform.state vpc dev vpc_id
@@ -385,17 +383,19 @@ The GCS backend supports multiple authentication methods:
 
 1. **Service Account Key File**: Specify the path to a service account JSON key file using the `credentials` parameter.
 2. **Default Credentials**: When no credentials are specified, the Google Cloud SDK default credentials are used.
-3. **Service Account Impersonation**: Use the `impersonate_service_account` parameter to impersonate a service account.
-4. **Workload Identity** (GKE): Automatically uses the workload identity when running in GKE with Workload Identity enabled.
+3. **Workload Identity** (GKE): Automatically uses the workload identity when running in GKE with Workload Identity enabled.
 
 ### GCS Backend Configuration Parameters
 
 - `bucket`: The GCS bucket name where Terraform state files are stored
 - `prefix`: Optional prefix for the state file path within the bucket
 - `credentials`: Optional path to a service account JSON key file
-- `impersonate_service_account`: Optional service account email to impersonate
 - `state_file`: Optional custom name for the state file (defaults to `default.tfstate`)
 
+:::note Current Limitations
+The `impersonate_service_account` parameter is parsed but not yet implemented. This feature is planned for a future release.
+:::
+
 ## Using `!terraform.state` with `static` remote state backend
 
 Atmos supports [brownfield configuration by using the remote state of type `static`](/core-concepts/components/terraform/brownfield/#hacking-remote-state-with-static-backends).