Merge pull request #5397 from richardcase/metadata_release28

🌱 chore: update metadata for v2.8.x release series

Author: k8s-ci-robot

metadata.yaml

@@ -59,3 +59,6 @@ releaseSeries:
   - major: 2
     minor: 7
     contract: v1beta1
+  - major: 2
+    minor: 8
+    contract: v1beta1