code: func (b deprecatedBackendShim) PrepareConfig(obj cty.Value) (cty.Value, tfdiags.Diagnostics) {
  	newObj, diags := b.Backend.PrepareConfig(obj)
  	return newObj, diags.Append(tfdiags.SimpleWarning(b.Message))
  }
docs: // PrepareConfig delegates to the wrapped backend to validate its config
  // and then appends shim's deprecation warning.