Kibana breaking changes

Details
Elastic Agents now use the Fleet Server to enroll agents, get agent policies, collect status information, and more. For more information, refer to #97206.

Impact
To run and manage Elastic Agents, use the Fleet Server instead of Kibana. For more information, refer to Fleet Server.

Details
The existing agents in Kibana are not migrated as part of the migration to Fleet. For more information, refer to #95789.

Impact
The existing agent API keys are invalidated and display as Inactive on the Agents page.

Details
The Explore underlying data context menu on dashboards is now disabled by default. For more information, refer to #98039.

Impact
To enable the Explore underlying data context menu, set xpack.discoverEnhanced.actions.exploreDataInContextMenu.enabled to true in kibana.yml.

Details
Geo containment alerts and the Geo threshold alert type have similar functionality, so the Geo threshold alert type has been removed. For more information, refer to #89632.

Impact
Use Geo containment alerts.

Details
When you upgrade to 7.12.0, Kibana uses a new algorithm to migrate saved objects to reduce the downtime caused by temporary issues, such as network errors or temporarily overloaded Elasticsearch clusters. With the new algorithm, Kibana now retries the failed migrations without user intervention, which allows the migration to continue after the issues are resolved. For more information, refer to saved object upgrade migrations.

Impact
Before you upgrade to 7.12.0, review the documentation for rolling back to a previous version of Kibana.

Details
In 7.12.0, Kibana introduces a major incompatibility with cross-cluster search. When one or more remote Elasticsearch clusters use a version prior to 7.12.0, Discover, Maps, Canvas, and Logs are unable to display the cluster data. For information about the cross-cluster search limitations in Kibana, refer to #94965.

Impact
To display the cluster data in Discover, load documents directly from _source.