Zoekt: Update replica feature documentation and UI visibility

Problem

Two minor issues were discovered during testing of the replicas feature:

1. The gitlab:zoekt:info rake task output still mentions the feature is experimental:

   Specify default number of replicas per namespace. Currently in development. See: https://gitlab.com/groups/gitlab-org/-/epics/19097:  1

2. The admin UI setting is hidden in the code: https://gitlab.com/gitlab-org/gitlab/-/blob/32fa31d984e5d5518e88ed48c9dc7bab4515ac00/ee/app/models/search/zoekt/settings.rb#L112

Proposal

1. Update the rake task output to remove the "Currently in development" message
2. Unhide the admin UI setting to make it accessible to administrators

Related

• Parent epic: &19097 (closed)
• Discovered in: &19097 (comment 2912420452)