Update v3 rolling upgrade instructions with minimum version requirement

Added note about minimum required cluster version for upgrades to 3.x.x.

Signed-off-by: Sokratis Papadopoulos <[email protected]>

Author: spapadop

_migrate-or-upgrade/rolling-upgrade/index.md

@@ -24,6 +24,9 @@ Before making any changes to your OpenSearch cluster, is it highly recommended t
 **Important**: OpenSearch nodes **cannot be downgraded**. If you need to revert the upgrade, then you will need to perform a new installation of OpenSearch and restore the cluster from a snapshot. Take a snapshot and store it in a remote repository before beginning the upgrade procedure. Rolling upgrades are **only supported between major adjacent versions**, for example, from OpenSearch 1.x to 2.x but not 1.x to 3.x.
 {: .important}
 
+**Important**: The minimum required cluster version for upgrades to `3.x.x` is `2.19.0`.
+{: .important}
+
 ## Performing the upgrade
 
 1. Verify the health of your OpenSearch cluster before you begin. You should resolve any index or shard allocation issues prior to upgrading to ensure that your data is preserved. A status of **green** indicates that all primary and replica shards are allocated. See [Cluster health]({{site.url}}{{site.baseurl}}/api-reference/cluster-api/cluster-health/) for more information. The following command queries the `_cluster/health` API endpoint: