MIAMI — The U.S. government began deporting Venezuelans held at immigration detention centers, transferring dozens to Mexico on Thursday even though many of them say they had entered the United States before the new immigration measures were announced. A group of about 150 Venezuelans who were in a detention center in Texas were placed in buses and driven to the border and forced to cross a pedestrian bridge that connects El Paso with Ciudad Juárez, where Mexican immigration officials were waiting. Jesus Rivero, 26, said that he and his brother Jonathan, 37, were among them and that in their c…