Bump fluxcd/flux2 to latest release (#5210)

Author: eks-distro-pr-bot

UPSTREAM_PROJECTS.yaml

@@ -105,8 +105,8 @@ projects:
     repos:
       - name: flux2
         versions:
-          - tag: v2.7.5
-            go_version: "1.25"
+          - tag: v2.8.3
+            go_version: "1.26"
       - name: helm-controller
         versions:
           - tag: v1.4.5