WASHINGTON, June 28 — Striking down an antitrust rule nearly a century old, the Supreme Court ruled today that it is no longer automatically unlawful for manufacturers and distributors to agree on setting minimum retail prices. The decision will give producers significantly more leeway, though not unlimited power, to dictate retail prices and to restrict the flexibility of discounters. Five justices said the new rule could, in some instances, lead to more competition and better service. But four dissenting justices agreed with the submission of 37 states and consumer groups that the abandonment of the old rule would lead to...