Write migration docs

We need to mention:

• Index migration using the migrationtool
• Migration is only officially supported from SG FLX, not SG classic
• ...

Upgrade sequence

1. Configure Search Guard to protect Eliatra Suite indices
2. Migrate indices
3. Rolling upgrade of nodes from ES to OpenSearch running Eliatra Suite compat edition

After all nodes are upgraded:

4. Upgrade Kibana to OpenSearch dashboards with Eliatra Suite plugin
5. Rolling upgrade of nodes from OpenSearch running Eliatra Suite compat edition to OpenSearch Eliatra Suite final edition (not necessary to be performed immediately, this can be delayed and combined with the next minor update)

Kibana / Dashboards

Our current understanding:

• When Kibana is used in a mixed cluster, one has to make sure by configuration that Kibana connects only to an Elasticsearch node
• Before the last Elaticsearch node is going to be shut off, Kibana needs to be shut down
• Afterwards start OpenSearch dashboards

To be clarified: Indices, index migration