SEOUL—South Korea’s top court upheld on Thursday a 20-year jail sentence for former President Park Geun-hye on graft charges that led to her downfall, bringing an end to the legal process and so for the first time raising the possibility of a pardon. Park became South Korea’s first democratically elected leader to be thrown out of office when, in 2017, the Constitutional Court upheld a parliament vote to impeach her over a scandal that also landed the heads of two conglomerates in jail. Park took office in 2013 as South Korea’s first woman president. She was brought down after being found guilty of colluding with a confidante to receive tens of billions of won from major conglomerates to help her family and fund non-profit foundations she owned. Her case has been heard in different courts, including a retrial in July last year, but the Supreme Court’s ruling on Thursday to …