By Brian Ellsworth FORT LAUDERDALE (Reuters) -A prosecutor on Monday urged jurors to sentence to death the gunman who killed 17 people in a mass shooting at a Florida high school in 2018. Nikolas Cruz, 23, pleaded guilty in October to the premeditated murder of 14 students and three staff members at Marjory Stoneman Douglas High School in Parkland, about 30 miles (48 km) north of Fort Lauderdale. Prosecutor Michael Satz told Broward County jurors on the first day of the penalty phase of the trial that Cruz committed “goal-directed planned, systematic murder — mass murder — of 14 students, an…