[20.10] update containerd binary to v1.6.18

update containerd binary to v1.6.18, which includes fixes for CVE-2023-25153
and CVE-2023-25173.

1.6.18 release notes: https://github.com/containerd/containerd/releases/tag/v1.6.18

> - Fix OCI image importer memory exhaustion (GHSA-259w-8hf6-59c2)
> - Fix supplementary groups not being set up properly (GHSA-hmfx-3pcx-653p)
> - Revert removal of /sbin/apparmor_parser check
> - Update Go to 1.19.6

full diff: https://github.com/containerd/containerd/compare/v1.6.17...v1.6.18

1.6.17 release notes: https://github.com/containerd/containerd/releases/tag/v1.6.17

> - Add network plugin metrics
> - Update mkdir permission on /etc/cni to 0755 instead of 0700
> - Export remote snapshotter label handler
> - Add support for default hosts.toml configuration

full diff: https://github.com/containerd/containerd/compare/v1.6.16...v1.6.17

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 52d667794fae15888285bc86029ae009f98d7d64)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

hack/dockerfile/install/containerd.installer

@@ -15,7 +15,7 @@ set -e
 # the binary version you may also need to update the vendor version to pick up
 # bug fixes or new APIs, however, usually the Go packages are built from a
 # commit from the master branch.
-: "${CONTAINERD_VERSION:=v1.6.16}"
+: "${CONTAINERD_VERSION:=v1.6.18}"
 
 install_containerd() (
 	echo "Install containerd version $CONTAINERD_VERSION"