WASHINGTON – The Trump administration is moving to end an agreement limiting how long migrant children can be kept in detention, the president’s latest effort to curb immigration at the Mexican border. A court fight is almost certain to follow, challenging the attempt to hold migrant families until asylum cases are decided. A current settlement overseen by the federal courts now requires the government to keep children in the least restrictive setting and to release them as quickly as possible, generally after 20 days in detention. Homeland Security officials say they are adopting their own regulations that reflect the “Flores agreement,” which has been in effect since 1997.