Provide an upgrade path for the .tasks index from 7.x to 8.x

[ywelsch]

The .tasks index is currently used to store information about past reindex / update-/delete-by-query requests, delete ML job actions, and index upgrade requests. As we currently only support index compatibility with the previous major ES version, upgrading across major versions might require a full reindex. For security and watcher indices, we have solved this problem by using aliases, marking indices temporarily as read-only, and then issuing a reindex, swapping the alias and removing the original index. For .tasks, this is a little bit more tricky as the reindex process uses .tasks, the index it tries to reindex.

[elasticmachine]

Pinging @elastic/es-distributed

[gwbrown]

This is addressed by #78951 - in that case, since the reindex is internal, we can safely wait for completion of the reindex, and thus there's no need to create a .tasks document for the reindex. I'm going to close this issue.