[Swift 6 Migration] Classes in Client/Generated conforming to `FeatureManifestInterface` should implement thread safety instead of suppressing warnings with `@unchecked Sendable`

[data-sync-user]

Please refer to https://mozilla-hub.atlassian.net/browse/FXIOS-13577 for the fullexplanation of the issue.

The types marked @unchecked Sendable need to either:

1. Conform correctly to Sendable (i.e. not contain mutable state)
2. Be synchronized to an actor (@MainActor, or another global actor)
3. Use another thread synchronization mechanism to manually conform to Sendable (e.g. a lock, a queue…)

┆Issue is synchronized with this Jira Task

[data-sync-user]

➤ Isabella commented:

Work has been approved and merged: mozilla/application-services#6963 ( https://github.com/mozilla/application-services/pull/6963|smart-link )

We are waiting for the next built to pull the update into our own repo.