Add changelog entry for EKS proxy-url (#9761)

Author: ashovlin

.changes/next-release/enhancement-eks-5115.json

@@ -0,0 +1,5 @@
+{
+  "type": "enhancement",
+  "category": "``eks``",
+  "description": "Add ``proxy-url`` to ``aws eks update-kubeconfig``."
+}