Update quay.io/skopeo/stable Docker tag to v1.21.0 (release-5.3) (#8702)

Update quay.io/skopeo/stable Docker tag to v1.21.0

| datasource | package               | from    | to      |
| ---------- | --------------------- | ------- | ------- |
| docker     | quay.io/skopeo/stable | v1.20.0 | v1.21.0 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/scripts/copy-images.sh

@@ -10,7 +10,7 @@ SKOPEO_BIN=skopeo
 if [ -n "$CI" ]; then
     SKOPEO_IMAGE=quay.io/skopeo/stable
     # renovate: datasource=docker depName=quay.io/skopeo/stable
-    SKOPEO_VERSION=v1.20.0-immutable
+    SKOPEO_VERSION=v1.21.0-immutable
     SKOPEO_BIN="docker run --rm -v $HOME/.docker/config.json:/tmp/auth.json ${SKOPEO_IMAGE}:${SKOPEO_VERSION}"
 fi