By Luc Cohen and Marianna Parraga NEW YORK/HOUSTON (Reuters) - A U.S. judge upheld the validity of Venezuelan state oil company PDVSA's 2020 bonds on Thursday, prompting a judge in another court to move towards the completion of an auction of shares in the parent of Venezuela-owned U.S. refiner Citgo Petroleum. The bonds are secured by a majority stake in Citgo, which is ultimately owned by Caracas-headquartered PDVSA. The state oil company defaulted on the bonds in 2019, putting the Houston-based refine ...