By Philip Pullella VATICAN CITY (Reuters) – Pope Francis apologised to the people of Democratic Republic of Congo and South Sudan on Sunday for having to cancel his visit because of walking problems, and said he was hopeful his condition would improve. The Vatican announced on Thursday that the July 2-7 trip had been postponed indefinitely because of the 85-year-old pontiff’s knee ailment, which has forced him to use a wheelchair for more than a month. “I feel great regret that I had to postpone this trip, which I am still very keen to make,” he said at his Sunday address before thousands of p…