don't merge mappings from source index

[rudolf]

In #70951 we added a function disableUnknownTypeMappingFields which would take a source index's mappings and the mappings calculated from all the registered saved object types and merge them in a special way: the mappings of any unknown type would be set to dynamic: false, properties: {}.

This ensured that we could still reindex any unknown saved object types even though the top-level mappings of the .kibana index has dynamic: strict.

For v7.x

In 7.x we only warn of unknown saved object types #105213 so we need to maintain this behaviour. However, it's only necessary to add mappings for unknown types, we don't need to merge in all the mappings from the source index as we do in https://github.com/elastic/kibana/blob/7.17/src/core/server/saved_objects/migrations/core/migration_context.ts#L172

This will help us avoid known issues where an upgrade fails because of problematic mappings in the source index.

For v8.x

In 8.x Kibana will refuse to start a migration when there are unknown saved object types #107678 We don't need any mappings from the source index, we should only be using the targetMappings calculated from the mappings of the registered saved object types so we can remove disableUnknownTypeMappingFields completely

[elasticmachine]

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

[rudolf]

thanks for raising this @romain-chanu 👍