How should I connect the ground wires for my new light fixture?
Having all those ground wires is common because many junction boxes are nonmetal. I'm assuming yours is. The actual ground should be the bare copper ground coming into the box with the feed. Connect that ground to the green wire attached to the box. Now the box is grounded so any brackets you attach to the box and fixtures will be grounded when assembled.
I like to ground the box with the bare copper wire attached directly to it with a 10-32 grounding screw and pigtail off that to the ground wires from the bracket and fixture. That way, there's no lose wires floating around in the box