Devon Still -- one of the most beloved athletes in pro sports -- is officially retiring from football ... telling TMZ Sports he's ready to move on to the next phase of his life. "I never wanted to be somebody who gives up on their dreams, but I don't want to be somebody who chases their dream too long and misses other opportunities." The defensive end entered the national spotlight in 2014 when he put his NFL career on hold to focus on helping his daughter, Leah, battle Stage 4 cancer.