Update quay.io/skopeo/stable Docker tag to v1.21.0 (#461)

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

Author: renovate[bot]

disconnected-csv/Dockerfile

@@ -1,5 +1,5 @@
 # renovate: datasource=github-releases depName=containers/skopeo
-FROM quay.io/skopeo/stable:v1.18.0
+FROM quay.io/skopeo/stable:v1.21.0
 
 LABEL version="1.0.0"
 LABEL repository="http://github.com/redhat-cop/github-actions"