[Reporting] Breaking: Remove setting for Reporting index

[tsullivan]

Summary

Address #101964 for Reporting

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

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

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 20eb83b
• Storybooks Preview
• Documentation Changes

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
reporting	40.8KB	40.9KB	+72.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[jloleysens]

Did not test locally but these changes look good to me 🍻

Couple of questions:

1. Do we have telemetry on how often this setting is being used in the wild?
2. How are we helping "broken" users to migrate?

[tsullivan]

Do we have telemetry on how often this setting is being used in the wild?

@jloleysens we do NOT have telemetry to see how often this setting is used. The meta issue on this is here: #82020 I presume this is a high-visibility breaking change as it affects other plugins as well.

How are we helping "broken" users to migrate

Spaces should be used when multitenancy is needed. The system indices were customizable in 7.x, which is the same release cycle that introduced Spaces.

From the meta issue:

Using saved-object management, a user is able to export all of the saved-objects from a legacy multitenant instance to a Space in the default tenant. However, there are currently some Kibana entities that can't be exported and imported using saved-object management. If we're going to no longer allow users to utilize legacy multitenancy, we should provide them a method of transitioning to Spaces, and as such, we'll need to ensure that users have a clear migration path.

This is an issue for Reporting: reports are not importable into Kibana. The only way a user will be able to view historical reports that were in a custom index is to download them to another form of storage before upgrading to 8.0.

[tsullivan]

Thread on this topic: #82020 (comment)