Newsletter: dashboard-changes migration / announcement page (Subscribers → Newsletter)

## Summary

As the modernized Newsletter dashboard rolls out, the information architecture changed — notably the standalone **Subscribers** entry now lives under **Jetpack → Newsletter → Subscribers**. Users who relied on the old Subscribers shortcut need a transitional migration/announcement page so they learn the new location before the old entry points go away. Design-owned; grew out of the Subscribers-menu/reframe discussion during the modernized-dashboards Call for Testing (tracked in Automattic/jetpack#49485 as item NL25).

## Scope

- [ ] **wp-admin** announcement page when `rsm_jetpack_ui_modernization_newsletter` is enabled — *in progress:* [#49496]() (replaces the silent "Subscribers ↗" Calypso shortcut with a "Subscribers moved → Jetpack → Newsletter" page; mirrors the Forms migration page [Automattic/jetpack#43705]()).
- [ ] **Jetpack Cloud** parity — matching announcement for the Cloud Subscribers surface.
- [ ] **Calypso** parity — consistent UI across wp-admin / Cloud / Calypso.
- [ ] **Design** sign-off on copy + layout.

## References

* Tracker: Automattic/jetpack#49485 (Modernized Dashboards wrap up), item NL25
* Visual precedent: Jetpack Forms migration page [Automattic/jetpack#43705]()
* In-flight implementation: [#49496]()

Start by reviewing the in-flight wp-admin implementation in issue #49496 and the Forms migration page in issue #43705. Compare the requested announcement behavior across wp-admin, Jetpack Cloud, and Calypso, then confirm the copy and layout with Design. Done means consistent Subscribers migration messaging in all three surfaces when the modernization flag is enabled.

Written by the indexing model from the issue text.