[Search Sessions] Update documentation to reflect disabled by default

[lukasolson]

Clearly document the removal of search sessions and the impact on saved objects. Ensure this removal is shown in breaking changes documentation. Remove sections about the search session from product documentation. Document the YML config workaround in a knowledge base article.

--- Edit ---

Update https://www.elastic.co/guide/en/kibana/current/search-sessions.html to reflect that this is now disabled by default, with instructions on how to update YML to opt in. Make sure other references in documentation also reflect this change.

Update https://www.elastic.co/guide/en/kibana/8.17/search-session-settings-kb.html with the new defaults.

[elasticmachine]

Pinging @elastic/kibana-data-discovery (Team:DataDiscovery)

[florent-leborgne]

@lukasolson just to make sure:

• Is it disabled by default starting with 9.0, or is it already the case? (If yes, since when?) Just so I update the right version(s) of the docs (or point you to it if you want to make the changes :))
• Is there a PR introducing this change that has a release notes entry already? If not, I'll need to update it manually to our release notes/breaking changes and could use a blurb that contains the corresponding details.
• Where/what is the setting to edit that is now set to false, and that users would need to edit? If it's mentioned in settings docs we should update it as well.

Thanks!

[lukasolson]

• Is it disabled by default starting with 9.0, or is it already the case? (If yes, since when?) Just so I update the right version(s) of the docs (or point you to it if you want to make the changes :))

Yes, starting with 9.0. (In serverless, it has been disabled by default since 8.9.0.)

• Is there a PR introducing this change that has a release notes entry already? If not, I'll need to update it manually to our release notes/breaking changes and could use a blurb that contains the corresponding details.

#203927 is the PR that disabled it by default (doesn't look like there is a release notes section). I believe it was manually added to the release notes: https://www.elastic.co/guide/en/elastic-stack/9.0/release-notes-kibana-9.0.0.html (there's a section in there).

• Where/what is the setting to edit that is now set to false, and that users would need to edit? If it's mentioned in settings docs we should update it as well.

https://www.elastic.co/guide/en/kibana/8.17/search-session-settings-kb.html The setting is the yml config setting data.search.sessions.enabled: true (to opt back in).

[florent-leborgne]

I'm hoping to get this covered with the following updates:

• Mention on Search sessions docs page that they're now disabled by default in addition to being deprecated + info on how to enable it again: Discover docs updates for 9.0 docs-content#943
• Updated Kibana settings docs to mention new false state by default: [Docs] Settings - Specify that search sessions are now disabled by default #216091
• 9.0 release notes also communicate about this change: [Docs] Kibana 9.0 release notes #215772

cc @lukasolson let me know if that will do

[lukasolson]

That looks great to me!