Give a two weeks written notice that you will be leaving and then leave. There is no reason to feel guilty, Starbucks will survive and so will your coworkers.

Come to terms with your guilt.

You are going to feel guilty, for all sorts of reasons. But it's important to remember that you need to put yourself and your future first, and sometimes that just doesn't include Starbucks. Your coworkers will manage. They, too, have the same agency that you do.

Don't let the guilt eat you up. Try to replace it with gratitude for the experience! It was great that you had the time you had, but as they say, "all good things come to an end," right? Let it be bittersweet, rather than soured by a sense of obligation. Giving them a two weeks notice is more than enough warning!