scalability: use CAPZ release branch for Azure test

Author: jackfrancis

config/jobs/kubernetes/sig-scalability/sig-scalability-periodic-azure.yaml

@@ -17,7 +17,7 @@ periodics:
     extra_refs:
     - org: kubernetes-sigs
       repo: cluster-api-provider-azure
-      base_ref: main
+      base_ref: release-1.20
       path_alias: "sigs.k8s.io/cluster-api-provider-azure"
     - org: kubernetes-sigs
       repo: cloud-provider-azure