[UA] Reindex searchable snapshot indices

[jloleysens]

We need to make sure that the reindexing logic in upgrade assistant handles reindexing of indices in searchable snapshots correctly.

(More details to come as this is still under investigation)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[sophiec20]

pending clarification from elasticsearch team regarding how to handle searchable snapshots during the upgrade @bytebilly

[bytebilly]

The current recommendation is to not reindex searchable snapshots, since they're read-only by design and they can be supported using the new N-2 approach.

[jloleysens]

@sophiec20 @bytebilly , thanks for the responses! If this is the recommendation, from what I can tell there are a few options:

(1) We should not list these as "CRITICAL" deprecations, ideally this non critical status would come directly from GET /_migration/deprecations... Not much changes for UA, we just need to add some logic NOT to offer to reindex these indices.

(2) Do we want to prompt users to consider this from within UA at all? Perhaps it can be removed entirely from GET /_migration/deprecations (zero work for UA)

(3) We do want to surface the option to reindex snapshots in some way... Given time constraints this likely means some kind of documentation where we guide users through unmounting, reindexing (possibly through UA), then mounting again... This would not be a great experience, but would be possible.

(4) ...something else

[bytebilly]

I don't have strong opinions on whether this should be handled in Kibana or on the ES API, the user experience will be the same. I'm slightly in favor of ES API, but please check with engineers and agree on implementation details.

[jloleysens]

I spoke with @dakrone who said they will be going with option (2).

removed entirely from GET /_migration/deprecations (zero work for UA)

I'll close this once that work is merged.