Ref arch docs: Do not disable auto-migration on praefect nodes

What does this MR do?

The instructions in 3k and higher reference architecture guides for use of Gitaly Cluster with Praefect service appears to carry config lines that disable all praefect auto migrations on all praefect hosts.

(The intention may have been to cover 2/3 praefect hosts, or non-praefect hosts, like the rails config lines?)

This appears unnecessary for a new install focused documentation, and also incorrect for Praefect, since it will refuse to start up due to missing relations in the database as a result of its initial migrations not running.

This change removes the lines that disable praefect auto migration from all Gitaly Cluster using reference architecture doc pages.

The original line was added via !58885 (merged)

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/215559 (internal link)

Example error due to all praefect nodes disabling auto-migration when following the reference guide:

{
    "method": "POST",
    "path": "/-/projects",
    "format": "html",
    "controller": "ProjectsController",
    "action": "create",
    "status": 500,
    …
    "exception.class": "ActionView::Template::Error",
    "exception.message": "2:accessor call: route repository accessor: consistent storages: query: pq: relation \"repositories\" does not exist.",
    "exception.backtrace": [
        "lib/gitlab/git/repository.rb:94:in `rescue in root_ref'",
        "lib/gitlab/git/repository.rb:89:in `root_ref'",
        "app/models/repository.rb:520:in `root_ref'",
        "lib/gitlab/safe_request_store.rb:12:in `fetch'",
        "app/models/repository.rb:649:in `head_commit'",
        "app/models/repository.rb:660:in `tree'",
        "app/models/repository.rb:1019:in `file_on_head'",
        "app/models/repository.rb:572:in `block in avatar'",
        "lib/gitlab/gitaly_client.rb:325:in `allow_n_plus_1_calls'",
        "app/models/repository.rb:571:in `avatar'",
        "app/models/project.rb:1447:in `avatar_in_git'",
        "app/models/project.rb:1451:in `avatar_url'",
        "app/models/concerns/avatarable.rb:31:in `avatar_url'",
        "app/helpers/page_layout_helper.rb:62:in `page_image'",
        "app/views/layouts/_head.html.haml:17",
        "app/views/layouts/application.html.haml:6",
        "app/controllers/application_controller.rb:128:in `render'",
        "app/controllers/projects_controller.rb:87:in `create'",
    ],
    …
}

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