Time for a lot of empathy and less explaining.
Instead of telling your daughter not to be upset, just listen. She *is* sad. Her friends are off on different adventures. Did she have some beloved playmates at her last school? Do try to schedule playdates on the weekends to give her some consistency.
I'll share with you my experience as a nanny-- the summer after preschool graduation, before kindergarten, can be very tough. They aren't in their familiar place and yet they are anticipating another huge change down the line and may be very unsure of what to expect. I've had plenty of kids who really didn't want to go forward into new experiences the summer before kindergarten--- too much unknown and too many variables. Home is their 'safe place' because graduating meant losing their other safe place. Some very big, 'unexplainable' meltdowns ensued, even about doing very mundane things they usually enjoyed like going to the park. Be prepared for this.
Knowing that this is very common, and that kids just don't process this transition in the way we might (with our big, experienced adult brains) expect... go forward gently this summer. Try to keep things as consistent and stable for her as you can. Listen to her fears and give empathetic consolation. 'Yes, you really miss your friends, you loved them very much'. Trying to change their perception may not work, nor will negating their feelings. It only frustrates them and makes them feel even more insecure because our little ones do need to know their feelings *are realistic and understandable*. If we as parents cannot relate to our child, who do they have to turn to with their fears?
My guess is that like many children, she'll enjoy kindergarten AFTER she actually gets a chance to do it. Once she's bonded with the teacher, learned the routine and had a couple weeks to find some new friends with common interests, you'll likely see her relax a bit more.
And FYI--maybe the week before kindergarten, use the YouTube "Mr. Rogers Goes to Kindergarten" as a tool. (Don't use it beforehand-- let her stay as present as possible this summer.) I found this video very grounded and comforting for some children and gives them somewhat of a 'virtual' tour before you go visit the new classroom, so she has a small idea of what to expect. Good luck!