[Reporting] Backport Task Manager integration to 7.x

[tsullivan]

Kibana version: 7.15

In #64853, we decided not to backport the change from master to 7.x because of the possibility that the user had pending reports that were added to the .reporting index by ESQueue in an earlier 7.x version. Such reports would not get picked up in 8.0 because there would be no corresponding task instance in Task Manager for those jobs.

This is not ideal since it puts a huge delay on the new and improved code from reaching users in a release. Based on the amount of time that Task Manager has been used for Reporting in the dev and test environments, we've seen that the implementation is stable.

We can make the switch to Task Manager into a non-breaking change by updating the data model of .reporting documents and add a new field which will be a keyword of the version of Kibana where the report was queued. In the Task Manager integration, we can search for older reports that are pending and schedule them with Task Manager so they eventually execute.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)

[tsullivan]

In the Task Manager integration, we can search for older reports that are pending and schedule them with Task Manager so they eventually execute.

This will be done in the reports:monitor task which is currently responsible for finding reporting jobs that have been in processing status for too long, and need to be rescheduled so they can be retried.

When touching on this code, it will be good to get a re-review of how it works and try to incorporate any lessons about having a monitoring task from the search sessions team.