WASHINGTON — After more than a decade in which the Supreme Court moved gradually toward more leniency for minors convicted of murder, the justices on Thursday moved the other way.The high court ruled 6-3 along liberal-conservative lines against a Mississippi inmate sentenced to life in prison without the possibility of parole for fatally stabbing his grandfather when the defendant was 15 years old. The case is important because it marks a break with the court’s previous rulings and is evidence of the impact of a newly more …