Add migration note for remote cluster settings (#33632)

The remote cluster settings search.remote.* have been renamed to
cluster.remote.* and are automatically upgraded in the cluster state on
gateway recovery, and on put. This commit adds a note to the migration
docs for these changes.

Author: jasontedor

docs/reference/migration/migrate_6_0/settings.asciidoc

@@ -86,3 +86,16 @@ they are replaced with `script.allowed_types` and `script.allowed_contexts`.
 The `discovery.type` settings no longer supports the values `gce`, `azure` and `ec2`.
 Integration with these platforms should be done by setting the `discovery.zen.hosts_provider` setting to
 one of those values.
+
+==== Cross-cluster search settings renamed
+
+The cross-cluster search remote cluster connection infrastructure is also used
+in cross-cluster replication. This means that the setting names
+`search.remote.*` used for configuring cross-cluster search belie the fact that
+they also apply to other situations where a connection to a remote cluster as
+used.  Therefore, these settings have been renamed from `search.remote.*` to
+`cluster.remote.*`. For backwards compatibility purposes, we will fallback to
+`search.remote.*` if `cluster.remote.*` is not set. For any such settings stored
+in the cluster state, or set on dynamic settings updates, we will automatically
+upgrade the setting from `search.remote.*` to `cluster.remote.*`. The fallback
+settings will be removed in 8.0.0.