Merge pull request #90731 from openshift-cherrypick-robot/cherry-pick-90099-to-enterprise-4.15

[enterprise-4.15]  Red Hat Managed Lokistack example misses to set the "serviceAccount" field

Author: jeana-redhat

observability/logging/logging-6.0/log6x-upgrading-to-6.adoc

@@ -347,6 +347,8 @@ metadata:
   name: instance
   namespace: openshift-logging
 spec:
+  serviceAccount:
+    name: <service_account_name>
   outputs:
   - name: default-lokistack
     type: lokiStack