Remove deprecated workspace_prefix in v0.8.0

[findkim]

Consul docs for deprecation notice https://github.com/hashicorp/consul/pull/11836 Initial bug ticket https://github.com/hashicorp/consul-terraform-sync/issues/442

Remove

driver "terraform-cloud" {
  workspace_prefix = "my-prefix-"
}

The new option

driver "terraform-cloud" {
  workspaces {
    prefix = "my-prefix-"
  }
}