[mauroservienti]

Adds an ADR for how ServiceControl picks the RavenDB search engine of its indexes and how developers and operators learn about an index rebuild. Each pull request in the stack records its own change. This ADR covers the reasoning that spans all of them and should outlive them: why the database setting is left to the operator, the four rules for an index's engine, and how a definition change is caught before and during an upgrade.

• Stop adding the Corax search engine setting to existing databases #6034: stop writing search engine settings to existing databases
• Use Lucene for new and rebuilt indexes #6035: new and rebuilt indexes use Lucene
• Report locked indexes and index rebuilds at start-up #6036: report locked indexes and rebuilds at start-up
• Approve RavenDB index definitions to catch changes that trigger a rebuild #6013: approval test for index definitions
• Document ServiceControl upgrades that rebuild RavenDB indexes docs.particular.net#8577: operator guidance for upgrades that rebuild indexes

The ADR's status is Proposed. Merge it after the stack, and set it to Accepted then, or update the rules first if review changes them.

Consequences lists three items that the upgrade guide of the release has to cover:

• A pending Corax replacement started by 6.20.0 to 6.21.2 is expected to be replaced by a Lucene one, losing its progress. Use Lucene for new and rebuilt indexes #6035 does not discuss this case and no test covers it.
• A LockedError index with a changed definition makes start-up fail.
• Database settings are no longer pinned to Corax, and a rebuilt index changes engine without operator action.

docs/README.md links the ADR under Decisions and rationale. The Platform engineering context index for ServiceControl is updated in Particular/Platform#1676 and will link this ADR once it is merged.