STAMFORD — Mayor David Martin is one of 60 city leaders from across the country who signed a letter last week to the Federal Communications Commission demanding the agency protect net neutrality. “A free and open Internet forms the backbone of the 21st century economy, and as leaders of local communities we are acutely aware of the threat to education, innovation and economic growth posed by the proposal,” the mayors wrote. The FCC is set to vote Thursday to dismantle 2015’s net neutrality agreement, which was put in place under the Obama administration to ensure equal access to the internet.