Dockerfile: update containerd binary to v1.7.21 (static binaries and CI only)

Update the containerd binary that's used in CI and static binaries

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

```markdown changelog
Update containerd (static binaries only) to [v1.7.21](https://github.com/containerd/containerd/releases/tag/v1.7.21)
```

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Dockerfile

@@ -196,7 +196,7 @@ RUN git init . && git remote add origin "https://github.com/containerd/container
 # When updating 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.
-ARG CONTAINERD_VERSION=v1.7.20
+ARG CONTAINERD_VERSION=v1.7.21
 RUN git fetch -q --depth 1 origin "${CONTAINERD_VERSION}" +refs/tags/*:refs/tags/* && git checkout -q FETCH_HEAD
 
 FROM base AS containerd-build

Dockerfile.windows

@@ -164,7 +164,7 @@ SHELL ["powershell", "-Command", "$ErrorActionPreference = 'Stop'; $ProgressPref
 ARG GO_VERSION=1.21.13
 ARG GOTESTSUM_VERSION=v1.8.2
 ARG GOWINRES_VERSION=v0.3.1
-ARG CONTAINERD_VERSION=v1.7.20
+ARG CONTAINERD_VERSION=v1.7.21
 
 # Environment variable notes:
 #  - GO_VERSION must be consistent with 'Dockerfile' used by Linux.

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.
-: "${CONTAINERD_VERSION:=v1.7.20}"
+: "${CONTAINERD_VERSION:=v1.7.21}"
 
 install_containerd() (
 	echo "Install containerd version $CONTAINERD_VERSION"