Sept 21 (Reuters) – S&P Global Ratings downgraded China’s long-term sovereign credit rating on Thursday, less than a month ahead of one of the country’s most sensitive political gatherings, citing increasing risks from its rapid build-up of debt. S&P’s one-notch downgrade to A+ from AA- comes as Beijing grapples with the challenges of containing financial risks stemming from years of credit-fuelled stimulus to meet ambitious government economic growth targets. “The downgrade reflects our assessment…