Removed Corax search engine settings are restored on existing databases at every start-up #6094

Describe the bug

At each start-up, including a start in maintenance mode, ServiceControl adds Indexing.Auto.SearchEngineType and Indexing.Static.SearchEngineType with value Corax to every database that has no such settings. The intent was a one-time pin for databases created before 6.20. The code cannot tell "never pinned" from "the operator removed the setting", so it also undoes an operator's choice.

RavenDB Studio removes a database setting whose value equals the server default. An operator who sets both keys to Lucene on a server whose default is Lucene therefore deletes the keys, and ServiceControl restores Corax at the next start. The "Migrating the whole database" procedure in the documentation produces exactly this.

Steps to reproduce

1. Create an error or audit database with ServiceControl 6.19 or earlier.
2. In RavenDB Studio, set Indexing.Auto.SearchEngineType and Indexing.Static.SearchEngineType to Lucene on the database. The server default is Lucene, so Studio removes both settings.
3. Start ServiceControl.
4. Both settings are back with value Corax.

Relevant log output

No response

Additional Information

• Affected versions: 6.20.0 to 6.22.1
• Fixed on master by Stop adding the Corax search engine setting to existing databases #6034

Backported to

• 6.22.2: Removed Corax search engine settings are restored on existing databases at every start-up #6090