Expire unused Brave Sync accounts

[fmarier]

We should expire Sync chains which haven't been used in a long time. This is useful for two reasons:

1. From a cost point of view, we should avoid paying for storage of data that will never be used again.
2. If a user loses all of their devices, then they have no way to request deletion of it. In order to somewhat limit the potentially unauthorized spread of their personal data, we should put an upper bound on how long we will retain unused data.

Expiry date: 12 months to start (discussed with @rebron)

Pre-requisites

• Server-side work: Expire unused Sync accounts go-sync#130
• Privacy policy change to mention the expiry: https://github.com/brave/brave-com/pull/2024
• Update the Sync FAQ to mention the expiry: https://github.com/brave/support/issues/5328
• Community post to announce the change: https://community.brave.com/t/notice-unused-sync-accounts-will-now-expire-after-one-year-of-inactivity/534779
• Tweet about the change (linking to the community post): https://twitter.com/brave/status/1763652094274220179

@PrivacyMatters suggested we communicate this 3 months ahead of time. We are planning to make the announcement on 2024-03-01.

Error: Your Sync chain has expired

This issue is about doing the client-side work to display a message to users when the client is connected to a Sync chain which has expired.

This is similar to what do when a client connects to a Sync chain which has been deleted:

Suggested error message (from https://docs.google.com/document/d/1X7glDHmeIjgipcPWn__S9rSxrI39P9rNw0RmuEuC3BI/edit#heading=h.7vqo8iomb0b3):

Your Sync chain expired [a week ago] due to inactivity. You can create a new Sync chain if you’d like to continue to use Sync.

Note: Sync chains automatically expire

[placeholder need mock-up]

In addition, we should have a note somewhere when creating a new Sync chain:

Note: Sync chains which have not been used in the last 12 months will automatically expire and the server data will be deleted.

Maybe adding it just above or just below the buttons here:

Brave Version

Targeting 1.66.x release

[fmarier]

cc @PrivacyMatters who has been asking for this for some time