Convert hook_search_api_db_query_alter() to an event

In #3023704: Convert hooks to events, we apparently forgot (I don’t think it was a conscious decision?) to convert hook_search_api_db_query_alter() of the Database Search sub-module, too.
Unless there’s any argument against doing that, I’d do that now. (Though, admittedly, it’s a bit unfortunate that people will now only get about six month’s advance notice for this change. So not sure we should actually remove the hook right away with the others. We might want to wait a bit at least for that one. (On the other hand, this is the time that every diligent module maintainer should keep a close eye on deprecation warnings anyways. So might be fine.))