Merge pull request #5394 from willie-yao/add-1.18-metadata

Add release 1.18 to metadata.yaml

Author: k8s-ci-robot

metadata.yaml

@@ -68,3 +68,6 @@ releaseSeries:
   - major: 1
     minor: 17
     contract: v1beta1
+  - major: 1
+    minor: 18
+    contract: v1beta1