cgroupv1: abort the update if Flatcar version does not support it

flatcar-postinst

@@ -244,11 +244,17 @@ if mountpoint -q /etc; then
   unshare -m sh -c "umount /etc && mkdir -p /etc/extensions && attr -R -r overlay.opaque /etc/extensions || true"
 fi
 
+MAX_NEXT_VERSION_ID=5000
+if [ "${NEXT_VERSION_ID%%.*}" -ge $MAX_NEXT_VERSION_ID && ! -f "/sys/fs/cgroup/cgroup.controllers" ]; then
+    echo "Flatcar version ${MAX_NEXT_VERSION_ID} or higher does not support cgroupv1 anymore. Aborting the update..."
+    exit
+fi
+
 # Keep old nodes on cgroup v1
 if [[ "${BUILD_ID}" != "dev-"* ]]; then
     if [ "${VERSION_ID%%.*}" -lt 2956 ]; then
 
-        # if there is an explicit setting, don't touch it. Otherwise...
+	# if there is an explicit setting, don't touch it. Otherwise...
         if ! grep -q systemd.unified_cgroup_hierarchy "${OEM_MNT}/grub.cfg" 2>/dev/null; then
             cat >>"${OEM_MNT}/grub.cfg" <<EOF