Add warning about prefix loss when migrating

Adds a warning about the consequences of switching to the `cloud` block, in terms of losing `prefix` support, and the impact that has on CLI usage.

website/docs/cli/cloud/migrating.mdx

@@ -82,3 +82,4 @@ terraform {
    }
  }
 ```
+~> **Warning:** Because the `cloud` provider does not support the `prefix` argument, once you migrate you will need to refer to workspaces by their full name when using the Terraform CLI. For example, rather than `terraform workspace select prod`, you would need to run the command `terraform workspace select my-app-prod`.