Zoekt: Create chatops command for managing replicas
## Background
This is an issue to allow us to specify the number of replicas on production without requiring a C2 CR.
## Proposal
Now, we should introduce a new chatops command / internal API to allow us changing the number of replicas on production (override). This is to enable this feature for `gitlab-org` before we assess storage impact and ready roll it out further.
I think that we should add:
```
/chatops run zoekt enabled_namespace set_replicas <root_namespace_id/group_path> <number_of_replicas/nil>
```
That would update `zoekt_enabled_namespace.replicas` override
## Context
- Examples:
- Admin API MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116650
- Chatops MR: https://gitlab.com/gitlab-com/chatops/-/merge_requests/368
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issue