Remote cluster settings

In Elasticsearch 8.15, the default value for skip_unavailable was changed from false to true. Before Elasticsearch 8.15, if you want a cluster to be treated as optional for a cross-cluster search, then you need to set that configuration. From Elasticsearch 8.15 forward, you need to set the configuration in order to make a cluster required for the cross-cluster search. Once you upgrade the local ("querying") cluster search coordinator node (the node you send CCS requests to) to 8.15 or later, any remote clusters that do not have an explicit setting for skip_unavailable will immediately change over to using the new default of true. This is true regardless of whether you have upgraded the remote clusters to 8.15, as the skip_unavailable search behavior is entirely determined by the setting on the local cluster where you configure the remotes.