April 21 (Reuters) - UnitedHealth opened the first-quarter earnings season for insurers with a strong performance, raising its annual profit forecast and surpassing Wall Street expectations for quarterly earnings ‌and revenue. The upbeat results boosted shares of the conglomerate, which has faced a series ‌of setbacks over the past two years, including rising medical costs, a cyberattack at its technology unit, and the killing ​of the head of its insurance unit that sparked a public backlash against the i ...