Display admin notice to US merchants still using the legacy/deprecated subscriptions functionality

[mattallan]

Project Thread: pgizk6-42-p2
Related Epic: #10401

This issue is being handled by the Chronos team as part of our Subscriptions Primary Offering project, as such, I've added this to our team's project board.

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• Targeted release: WooPayments 9.1
• Copy for these notices can be found in the google doc linked here (paJDYF-gCJ-p2#comment-28011)

There's a small number of US merchants using a legacy implementation of our Stripe Billing feature.

Back in 2023, we attempted to move merchants off this feature via an email campaign, however that was not 100% successful. With the release of WooPayments 9.0, we will be attempting to send another updated email therefore the idea of this issue is to follow-up this latest email with an admin notice on subscription admin pages and an inbox note.

Acceptance Criteria

• Display an admin notice on Subscriptions list and settings admin pages to US merchants with active legacy Stripe Billing subscriptions:
  • No WooCommerce Subscriptions extension active.
  • Existing Stripe Billing subscriptions.
• Display an WooCommerce Admin note to US merchants also with active legacy Stripe Billing subscriptions
• Installing WooCommerce Subscriptions removes all notices