Move organization General settings to admin area

What does this MR do and why?

Moves the organization "General settings" page from the organization sidebar into the organization admin area. It now lives at /o/:org/admin/settings/general, served by a new Admin::Organizations::SettingsController and gated by the org_admin_area release flag plus the :access_organization_admin_area ability, rather than the old org_pages flag and :update_organization ability.

This puts organization-level configuration alongside the dashboard and user administration, so all administrative surfaces sit under one gated area instead of the general org sidebar.

The old route, controller, view, sidebar item, and page entrypoint are removed, and a new Settings menu is added to the organization admin sidebar. The shared Vue settings app is reused unchanged; only the Rails page wiring moved. The outside-admin organization Settings menu is kept because EE still adds an Artifact Registry item under it, but in CE it no longer carries the General item.

The whole organization feature (and this admin area) remains behind default-off release flags, so this is not user-visible by default.

Screenshots or screen recordings

Before	After

No visual change: the same Vue settings app renders at the new admin-area URL.

How to set up and validate locally

1. Enable the organization release flags in the Rails console:

   Feature.enable(:org_stage_experimental)
   Feature.enable(:org_stage_beta)

2. Visit /o/<your-org>/admin/settings/general as an organization owner or instance admin.
3. Confirm the General settings render, and that the old /o/<your-org>/-/settings/general path no longer resolves.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.