Support indexing all new namespaces with Zoekt for SM and GitLab.com

Automatically enable indexing for all newly created paid groups on GitLab.com so we don't need to manually roll out for new customers. We have this logic already for Elasticsearch so the implementation can be similar.

By this point we may need to implement slightly more sophisticated logic for managing shard sizes so that new groups are assigned to less busy shards.

Update from !154272 (comment 1939193637)

Search::Zoekt::SchedulingService has a temporary task for dot_com_rollout for GitLab.com and create_zoekt_indexed_namespace_for_self_managed for self managed instances.

This issue should explore what the future of those tasks are prior to GA.