MONTREAL — Many Canadians go to their local outdoor hockey rink to skate and live the dream of scoring that winning Stanley Cup goal. But this quintessential slice of Canadian life in wintertime is slowly melting away because of global warming, says a collection of young environmentalists. ‘‘A bunch of us are fairly avid hockey fans and hockey players and we noticed that we were waiting longer and longer every year to play, and then we started phoning rink operators and were told the same story,’’ said Mike Hudema of Global Exchange. Hudema, a 28-year-old from Edmonton, Alta., said the...