Maine needs to increase the size and skill level of its workforce to keep businesses in the state and attract new companies, three development groups said Thursday. A report issued jointly by the Maine State Chamber of Commerce, the Maine Development Foundation and Educate Maine said the state’s quality of life, its workforce and its natural resources are all important factors that keep businesses in the state and attract others. But the report, “Making Maine Work 2018,” said the state’s workforce is shrinking because of an aging population and because too many workers lack the skills needed in the current economy.