Swift 6.1

[dfabulich]

We're using @preconcurrency import SkipFirebaseMessaging (Messaging should be Sendable but isn't) and @preconcurrency import SkipFirebaseFirestore (Firestore and CollectionReference should be Sendable but aren't).

We're also making our NotificationDelegate Sendable, so its methods can use @MainActor, and we're using @preconcurrency UNUserNotificationCenterDelegate so Swift doesn't complain that our @MainActor implementations don't fulfill the protocol.

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[dfabulich]

I filed skiptools/skip-firebase#87 so we don't have to add @preconcurrency to our imports. But @preconcurrency works, too.