JOHANNESBURG (AP) — In South Africa ’s oldest township, volunteers in wetsuits jump into the thigh-deep water of the polluted Jukskei River to untangle a net that’s designed to trap garbage but damaged by heavy rains. Without the nets, the shacks of low-lying Alexandra on the outskirts of Johannesburg could face disastrous flooding. World leaders with the Group of 20 rich and developing nations will meet this weekend in Johannesburg for the bloc’s first summit in Africa. Host South Africa wants