chore(deps): update docker.io/appuio/oc docker tag to v4.21

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

Author: renovate[bot]

Dockerfile

@@ -7,7 +7,7 @@ RUN make test && make build
 
 FROM ghcr.io/vshn/appcat-cli:latest AS appcat-cli
 # Runtime
-FROM docker.io/appuio/oc:v4.20
+FROM docker.io/appuio/oc:v4.21
 
 COPY --from=appcat-cli /bin/appcat-cli /bin/appcat-cli
 COPY --from=build /src/k8ify /bin/k8ify