Fix CCS cluster compatibility (#2028)

Updates the upgrade guide to clarify BWC for CCS. Based on [the CCS docs](https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-cross-cluster-search.html#ccs-supported-configurations), you can only search back one minor version.  This means a local 8.1 cluster can only search remote 8.0+ clusters.

Relates to elastic/kibana#125138 and #1970

Author: jrodewig

docs/en/install-upgrade/upgrading-stack.asciidoc

@@ -16,7 +16,7 @@ to give you flexibility in scheduling the upgrade.
 .Remote cluster compatibility
 [NOTE]
 ====
-If you use {ccs}, note that {version} can only search remote clusters running 7.17 or later. 
+If you use {ccs}, note that {version} can only search remote clusters running the previous minor version or later. 
 For more information, see {ref}/modules-cross-cluster-search.html[Searching across clusters].
 
 If you use {ccr}, a cluster that contains follower indices must run the same or newer version as the remote cluster.