Remove Alerting destinations concept and destination migrator for OpenSearch 3.0

[lezzago]

In Alerting 1.x and before, there was a concept of Alerting destination with its own set of APIs. In OpenSearch 2.x, we have deprecated Alerting destinations and introduced the Notification plugin and help migrate existing Alerting destinations to OpenSearch Notification plugin.
With OpenSearch 3.0 coming, we need to remove the deprecated Alerting destination APIs and the migration job (ref) that the Alerting plugin runs.

Similarly this work would also need to happen on Alerting dashboards for any old support for Alerting destinations.