test/e2e: bump Flatcar image to latest Stable

Signed-off-by: Mathieu Tortuyaux <[email protected]>

Author: tormath1

test/e2e/data/e2e_conf.yaml

@@ -186,7 +186,7 @@ variables:
   SSH_USER_MACHINE: "ubuntu"
   EXP_KUBEADM_BOOTSTRAP_FORMAT_IGNITION: "true"
   # The Flatcar image produced by the image-builder
-  OPENSTACK_FLATCAR_IMAGE_NAME: "flatcar-stable-3975.2.0-kube-v1.30.1"
+  OPENSTACK_FLATCAR_IMAGE_NAME: "flatcar-stable-4081.2.0-kube-v1.30.1"
   # A plain Flatcar from the Flatcar releases server
   FLATCAR_IMAGE_NAME: "flatcar_production_openstack_image"