BOSTON, Oct 5 (Reuters) - The yield starvation diet for American investors is over. Investors who quit products with ultra-low yields are hungry again for certificates of deposit and money market funds as interest rates surge to levels unseen since 2011. The shift in gravity on interest rates represents a return to normalcy after nearly a decade of rock-bottom yields. “Cash investors have been numb to yields,” said Pete Crane, president of money fund research firm Crane Data. “This money...