Promote NPD image v0.8.21

Author: wangzhen127

registry.k8s.io/images/k8s-staging-npd/images.yaml

@@ -17,3 +17,4 @@
     "sha256:50c4659db921d912a3b931300d1a1ef5d3c53dda896db6bc234f35673c713bb5": ["v0.8.18"]
     "sha256:a28d0bd3f6ecc9648ecedd7135c31c31751e340017cc9a663911f7b064a8a010": ["v0.8.19"]
     "sha256:52f0618e9bc2cea60b7aa9d9c962a0b84dace6ef3cff62d1e1a29c92ba1c4177": ["v0.8.20"]
+    "sha256:28312499057bd4ea928e85ee32e340723f64741c5cf02614ef1249cbccc1b140": ["v0.8.21"]