text: Title: [Alerting][7.x] Add migration that copies existing rule saved object ids to another location in the saved object
  Body: Relates to https://github.com/elastic/kibana/issues/100067
  
  To prepare for the upcoming saved object ID migration, we need to copy over the existing ID to a new location in the saved object for every rule.
  
  Once the migration happens in 8.0, there is no way to determine the legacy ID given the new, migrated ID. This is a problem for [our code that searches against the event log](https://github.com/elastic/kibana/blob/master/x-pack/plugins/alerting/server/rules_client/rules_client.ts#L455) as the event log will contain references to legacy IDs and we need to start searching across both IDs.
  
  See [the PoC PR](https://github.com/elastic/kibana/pull/107611) for more insight into how this might work.