message: add warning when using old enum behaviour
  prop(src): add warning when using old enum behaviour
diff: @@ -213,6 +213,7 @@ export function _buildPropMetadata(input: DecoratedPropertyMetadata) {
  } else {
  // old behaviour
  // TODO: remove in typegoose 7.0
  + logger.warn('Old Enum Behaviour is used, please upgrade to the new one');
  rawOptions.enum = Object.keys(enumOption).map((propKey) => enumOption[propKey]);
  }
  }
  