WASHINGTON — Millions of Americans who rely on Social Security can expect to receive their biggest payment increase in years this January, according to projections released Thursday by the trustees who oversee the program.But older Americans shouldn't get too excited.The increase is projected to be just 2.2 percent, or about $28 a month for the average recipient. Social Security recipients have gone years with tiny increases in benefits. This year they received an increase of 0.3 percent, after getting nothing last year.Some good news for seniors: The trustees project that Medicare Part B premiums will remain unchanged next year.