WILMINGTON, Del. — The old adage about looking a gift horse in the mouth may have a corollary regarding gift cards issued by retailers: Use it or lose it.In the latest case of gift card holders caught up in the bankruptcy of a major retailer, attorneys on Wednesday outlined a plan under which some holders of RadioShack gift cards would be paid in full for their outstanding balances. Others who hold the electronic retailers’ gift cards, however, would be left with pennies on the dollar, if anything.The plan is part of a …