Update Helm release rancher to v2.6.4

This MR contains the following updates:

Package	Update	Change
rancher (source)	patch	2.6.3 -> 2.6.4

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k8s/clusters/apps/rancher.yml

@@ -28,7 +28,7 @@ spec:
       source:
         repoURL: https://releases.rancher.com/server-charts/stable
         chart: rancher
-        targetRevision: 2.6.3
+        targetRevision: 2.6.4
         helm:
           values: |
             fullnameOverride: rancher