WASHINGTON — The Supreme Court’s conservative majority on Wednesday signaled it would uphold Mississippi’s 15-week ban on abortion and may go much further to overturn the nationwide right to abortion that has existed for nearly 50 years.The fate of the court’s historic 1973 Roe v. Wade decision legalizing abortion throughout the United States and its 1992 ruling in Planned Parenthood v. Casey, which reaffirmed Roe, probably won’t be known until next June.But after nearly two hours of arguments …