LONDON (Reuters) - Britain's 24 million adult gamblers will not be able to use credit cards to make bets under a new rule designed to prevent consumers from building up too much debt, sending shares in betting companies sharply lower.The Gambling Commission said on Tuesday the ban, which comes into effect on April 14, will apply to all online and offline gambling products apart from over-the-counter lottery tickets.Research by the commission classes 22% of online gamblers who use credit cards as..