admin/argocd: step 1/3 upgrade from 2.5 to 2.6

Related to swh/infra/sysadm-environment#4731

admin/argocd-installation-application.yaml

@@ -9,7 +9,7 @@ spec:
   source:
     repoURL: 'https://github.com/argoproj/argo-cd.git'
     path: manifests/ha
-    targetRevision: v2.5.2
+    targetRevision: v2.6.13
 
     directory:
       recurse: false