Dockerfile: update containerd binary to v1.7.21

Update the containerd binary that's used in CI

- full diff: containerd/containerd@v1.7.20...v1.7.21
- release notes: https://github.com/containerd/containerd/releases/tag/v1.7.21

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

Dockerfile

@@ -1,7 +1,7 @@
 # syntax=docker/dockerfile-upstream:master
 
 ARG RUNC_VERSION=v1.1.14
-ARG CONTAINERD_VERSION=v1.7.20
+ARG CONTAINERD_VERSION=v1.7.21
 # containerd v1.6 for integration tests
 ARG CONTAINERD_ALT_VERSION_16=v1.6.33
 ARG REGISTRY_VERSION=v2.8.3