Bump fluxcd/notification-controller to latest release (#5160)

Author: eks-distro-pr-bot

UPSTREAM_PROJECTS.yaml

@@ -117,8 +117,8 @@ projects:
             go_version: "1.25"
       - name: notification-controller
         versions:
-          - tag: v1.7.5
-            go_version: "1.25"
+          - tag: v1.8.2
+            go_version: "1.26"
       - name: source-controller
         versions:
           - tag: v1.7.4